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Technician Handbook 874 Engine Control Systems II O2 and A/F Sensor Diagnosis OBD II vehicles require two exhaust sensors: one before and one after the catalytic converter The A/F or O2 sensor before the catalytic converter is used by the ECM to adjust the air/fuel ratio and is in the S1 position (AFS B1 S1) The O2 sensor after the catalytic converter is used for catalytic converter efficiency control and monitoring; and is in the S2 position (O2S B1 S2) There are several types of oxygen sensors The two most common are listed below: •  Narrow range oxygen sensor, typically called an oxygen (O2) sensor •  Wide range oxygen sensor, typically called an air/fuel ratio (A/F) sensor Technical Training 61 Technician Handbook 874 Engine Control Systems II Oxygen Sensor Construction and Operation The oxygen sensor, sometimes referred to as heated oxygen sensor, HO2S or O2S, has been in service the longest It is made of zirconia (zirconium dioxide), platinum electrodes, and a heater There are two types in use: the Cup type and the Planar type They vary slightly in construction and operation, but cannot be interchanged Refer to the Repair Manual for specifics The oxygen sensor generates a voltage signal based on the amount of oxygen in the exhaust compared to the atmospheric oxygen •  When exhaust oxygen content is high (lean), oxygen sensor voltage output is low •  When exhaust oxygen content is low (rich), oxygen sensor voltage output is high •  During normal operation, the voltage signal cycles from 100 mV to 900 mV (0.1 V to 0.9 V) The zirconia element has one side exposed to the exhaust stream while the other side is open to the atmosphere Each side has a platinum electrode attached to the zirconium dioxide element The platinum electrodes conduct the voltage generated Contamination or corrosion of the platinum electrodes or zirconia elements will reduce the voltage signal output Oxygen sensors signal circuits can be tested with a DVOM Refer to the Repair Manual for the correct diagnostic procedure NOTE 62 Oxygen sensors from different vehicles are NOT interchangeable Verify correct part numbers, tag colors, electrical connector colors and shapes, etc before installing new parts Technical Training Technician Handbook 874 Engine Control Systems II Oxygen Sensor Heater The oxygen sensor cannot produce an accurate voltage signal until it reaches a minimum operating temperature of 750 degrees F (400 degrees C) It must reach that temperature quickly and stay at that temperature for effective operation To help the oxygen sensor reach its operating temperature quickly, the ECM turns on current flow through a heating element inside the sensor This element heats up as current passes through it The ECM controls the circuit based on engine coolant temperature and engine load (determined from the MAF sensor signal) The oxygen sensor heater circuit uses approximately amperes and is generally turned OFF once the engine reaches normal operating temperature Typically, the heater will be turned ON at idle or in decel fuel cut conditions NOTE Unlike the A/F sensor heater being pulsewidth modulated (PWM), the O2 sensor heater typically is not Typically, when any O2 sensor heater DTCs are present, the ECM will turn OFF the A/F sensor heater as part of the fail-safe mode The fail-safe mode will continue until the ignition switch is turned OFF Because proper operation of the oxygen sensor depends on correct sensor temperature, the heater should always be checked when testing the sensor A DVOM can be used to test the O2 sensor heater operation Refer to the Repair Manual for the correct diagnostic procedure and values Technical Training 63 Technician Handbook 874 Engine Control Systems II 64 NOTE Some 2004 and later model vehicles will utilize a relay in the O2 sensor heater circuit Refer to the Repair Manual and the Electrical Wiring Diagram for the correct diagnostic procedure, values and wiring diagram NOTE With the ignition ON, engine OFF, voltage at the ECM side of the heater circuit should be battery voltage With the engine ON, voltage at the ECM side of the heater circuit should be less than battery voltage due to the ECM grounding the circuit and the oxygen sensor heater load Also, some types of O2 sensor heater circuits (most Planar) are pulsewidth modulated (PWM) Comparing the O2 sensor heater voltage readings at ignition ON, engine OFF and engine ON to a similar known good vehicle at the same temperature can also aid in diagnosis Technical Training Technician Handbook 874 Engine Control Systems II A/F Sensor Construction and Operation The A/F sensor, sometimes referred to as the AFR sensor or air/fuel ratio sensor, looks like an oxygen sensor and serves the same purpose, but it is different in construction and operation Instead of a varying voltage output, the A/F sensor changes its current (amperage) output in relation to the amount of oxygen in the exhaust stream A detection circuit in the ECM uses this amperage to create a voltage signal that varies with the oxygen content of the exhaust gases •  At stoichiometry, there is no current flow and the detection circuit outputs 3.3 volts •  When exhaust oxygen content is high (lean), a positive current is produced and the detection circuit outputs a voltage above 3.3V •  When exhaust oxygen content is low (rich), a negative current is produced and the detection circuit outputs a voltage below 3.3V These sensors detect A/F ratios over a wider range, allowing the ECM to more accurately control fuel injection and reduce emissions Because of its nature, the A/F sensor signal circuit cannot be tested with a DVOM Instead, the Techstream data list parameter is used to read the A/F sensor signal Refer to the Repair Manual for the correct diagnostic procedure NOTE Technical Training A/F sensors from different vehicles are NOT interchangeable Verify correct part numbers, tag colors, electrical connector colors and shapes, etc before installing new parts 65 Technician Handbook 874 Engine Control Systems II Approximately, 0.4 volt differential is continuously applied across terminals AF + and AF - (AF +: 3.3V, AF -: 2.9V) of the A/F sensor signal circuit The zirconia element is connected in series with these two terminals and generates an electromotive force based on the air/fuel ratio The electromotive force or voltage differential determines the strength and direction of current flow in the circuit When Air/Fuel ratio is at stoiciometric, electromotive force is equal to about volts AF- (2.9 volts contant) + electromotive force volts = 3.3 volts This is equal to the voltage supplied by AF+ (3.3 volts constant) If AF- and AF+ equal each other there is no current flow in the circuit and is interpreted as a balanced Air/ Fuel Ratio or Stoiciometric 66 Technical Training Technician Handbook 874 Engine Control Systems II When the electromotive force is less than approx 0.4 V, voltage is higher at AF+ than at AF- This causes current to flow from AF+ to AF- in accordance with the voltage differential Technical Training 67 Technician Handbook 874 Engine Control Systems II If an electromotive force is higher than approx 0.4 V, voltage is higher at AF- than at AF+ This causes current to flow from AF- to AF+ in accordance with the voltage differential 68 Technical Training Technician Handbook 874 Engine Control Systems II A/F Sensor Heater A/F sensors operate at temperatures even hotter than O2 sensors, approximately 1200 degrees F (650 degrees C) The A/F sensor heater serves the same purpose as the O2 sensor heater, but there are some very important differences A/F sensors require a much higher operating temperature than O2 sensors and must heat up to operating temperature very fast (within seconds) so: •  Some vehicles use an A/F Relay (turned on at the same time as the EFI Relay) A relay is required because the A/F sensor heater circuit carries up to 9.9 amperes (versus amperes for oxygen sensor heater) to produce the additional heat needed by the A/F sensor Refer to the Repair Manual for correct A/F sensor heater operating range values •  This heater circuit is pulsewidth modulated (PWM) When cold, the duty ratio is high •  The heater may be ON under normal driving conditions to maintain proper A/F sensor operating temperature Typically, when any A/F sensor heater DTCs are present, the ECM will turn OFF the A/F sensor heater as part of the fail-safe mode The fail-safe mode will continue until the ignition switch is turned OFF Technical Training 69 Technician Handbook 874 Engine Control Systems II A/F Sensor Heater (cont’d) NOTE 70 Because proper operation of the A/F sensor depends on correct sensor temperature, the heater should always be checked when testing the sensor A DVOM or oscilloscope can be used to test the A/F sensor heater operation Refer to the Repair Manual for the correct diagnostic procedure and values With the ignition ON, engine OFF, voltage at the ECM side of the heater circuit should be battery voltage With the engine ON, voltage at the ECM side of the heater circuit should be less than battery voltage (pulsewidth modulated [PWM]; varies with sensor temperature) due to the ECM grounding the circuit and the A/F sensor heater load Comparing the A/F sensor heater voltage readings at ignition ON, engine OFF and engine ON to a similar known good vehicle at the same temperature can also aid in diagnosis Technical Training Technician Handbook 874 Engine Control Systems II Catalytic Converter Overview Catalytic converter failures generally fall in the category of physical damage or catalyst failure Physical damage usually can be visually identified – cracks, dents, etc Internally, the structure can be cracked, broken, or melted Where high heat may lead to catalyst failure, the engine and related systems need to be thoroughly checked Catalyst performance before OBD II was determined differently in many states by test equipment OBD II systems can determine catalyst performance Performance deteriorates in many cases when the catalyst becomes coated with foreign materials Contaminated fuel, sealants, or coolant can all affect catalyst performance The sub (S2) O2 sensor is used to adjust the oxygen level in the catalytic converter to achieve the best catalytic converter efficiency possible As a catalytic converter deteriorates, its ability to store oxygen is also reduced During conversion, the stored oxygen is rapidly depleted The sub O2 sensor detects this and, within a very limited range, the ECM will reduce the amount of fuel injected, providing more oxygen to the converter Oxygen levels build up, driving the O2 signal downward At a predetermined point, fuel control will return to stoichiometric A/F ratio When this happens, oxygen again will be depleted, driving the sub O2 signal upward, and the cycle will repeat itself The rate at which this cycle repeats depends on how much the catalyst has deteriorated, engine load, and the amount of correction as determined by the ECM's fuel control programming 76 Technical Training Technician Handbook 874 Engine Control Systems II Catalytic Converter Monitor The diagnostic system measures the oxygen storage capacity (OSC) of the catalyst This is based on the correlation between catalyst conversion efficiency and oxygen storage capacity Catalyst efficiency is monitored by comparing the pre-catalyst O2 or A/F sensor output signal with the signal received from the post-catalyst O2 sensor The ECM uses voltage variations between these sensors to measure the catalyst performance When the converter is operating properly, the post-catalyst sensor is significantly less active than the pre-catalyst sensor This is because the converter stores and releases oxygen as needed during its reduction and oxidation processes, so the post-catalyst sensor is exposed to exhaust gases with very little variation in oxygen levels After the engine and catalyst are warmed up and the readiness monitor enabling conditions are met, the ECM will run the Catalyst monitor Catalyst warm-up is determined by a calculation in the ECM’s internal programming Engine load, engine coolant temperature, and time are the primary factors used to determine catalyst temperature Later model vehicles equipped with A/F sensors in the S1 position utilize Active Air/Fuel Ratio Control to monitor the efficiency of the catalytic converter, A/F sensor, and O2 sensor See the Active Air/Fuel Ratio Control topic in this section for more information Catalytic Converter Efficiency Below Threshold DTCs Technical Training P0420: Catalyst System Efficiency Below Threshold (Bank 1) (two-trip) P0430: Catalyst System Efficiency Below Threshold (Bank 2) (two-trip) 77 Technician Handbook 874 Engine Control Systems II There are three distinct Techstream tests that can be used to check the exhaust sensors and catalyst efficiency These tests are typically used for early model vehicles with Passive type monitors These tests are not the same for an Active type monitoring system which is explained at the end of this section O2 and A/F Sensor Activation Check the O2 Sensor monitor Test Results and Test Details prior to and after diagnosis and repair to confirm that the repair has been effective Because proper operation of the A/F and O2 sensors depends on correct sensor temperature, the heater should always be checked when testing the sensor When trying to determine the cause of an A/F or O2 sensor issue, check the Freeze Frame data and duplicate the conditions Use the Technical Information System (TIS) for Repair Manual (RM) and Electrical Wiring Diagram (EWD) information, and look for applicable Service Bulletins (SB) Prior to determining if an A/F or O2 sensor is faulty, it is essential that the A/F and/or O2 (exhaust) sensors be at operating temperature Prior to testing the exhaust sensors, run the engine in Park between 2,500 and 3,000 RPM for approximately minutes This will ensure that all exhaust sensors are at operating temperature and ready for testing 78 Technical Training Technician Handbook 874 Engine Control Systems II O2 and A/F Sensor Test For A/F (S1) sensor equipped vehicles: After activating the sensors with the proper engine racing pattern, alternate the engine speed between 2,000 and 3,000 RPM in 2second intervals while observing the A/F and O2 sensor parameter readings with the Techstream The next image shows a graphed snapshot of this test For O2 (S1) sensor equipped vehicles: After activating the sensors with the proper engine racing pattern, run the engine speed at approximately 2,500–3,000 RPM while observing the O2 sensor parameter readings with the Techstream Refer to the Repair Manual for the correct responses and values Comparing the responses of the exhaust sensors from one bank to the other (if applicable) or to the exhaust sensor responses from a similar known good vehicle can also in diagnosis Technical Training 79 Technician Handbook 874 Engine Control Systems II O2 and A/F Sensor Activation and Test Snapshot In the illustration, the engine speed was alternated between 2,000 and 3,000 RPM in 2-second intervals while observing the A/F and/or O2 sensor parameter readings with the Techstream on a known good vehicle in Park with the engine at operating temperature The A/F and O2 sensors and catalytic converters have passed this test because all sensors fluctuated, and the O2 (S2) sensors did not fluctuate up and down frequently If either voltage output of the A/F or O2 sensor does not fluctuate, or if there is noise in the waveform of either sensor, this may indicate a malfunctioning sensor If the voltage outputs of both sensors on the same bank, or all sensors on both banks if applicable, remain lean or rich, the air/fuel ratio may be extremely lean or rich If this is the case, perform an A/F Control active test with the Techstream See the O2 and A/F Sensor Test 2: Injector Volume or A/F Control Active Test portion of this section for more information On some vehicles, the Catalyst Efficiency Below Threshold DTC (P0420, P0430) detection conditions may be so small that a malfunction may not be observable with these tests Typically, if the MAF sensor, A/F sensor, O2 sensor, and fuel trims are within specifications, and a P0420 or P0430 DTC continues to set after clearing the DTCs and re-running the monitor, this may indicate a faulty catalytic converter Also, always check the Catalyst Efficiency monitor Test Details after running the monitor If a Test Result is close to the Min Limit, this may result in an intermittent MIL ON condition If the Test Result is well within the Min and Max Limits after making a repair attempt, clearing DTCs and re-running the monitor, this probably means that the repair attempt was successful See the Readiness Monitor Test Details portion of the OBD section in this course for more information NOTE 80 If there are engine issues such as misfires, faulty injectors, incorrect timing, etc., these may cause the catalytic converter to malfunction prematurely Ensure that the engine is running correctly and that no misfires, fuel trim, or other engine issues are present before diagnosing the catalytic converter Technical Training Technician Handbook 874 Engine Control Systems II O2 - A/F Sensor & Catalyst (OSC) Test 2: Injector Volume or A/F Control Active Test Check the O2 Sensor & Catalyst Efficiency monitor Test Results and Test Details prior to and after diagnosis and repair to confirm that the repair has been effective Because proper operation of the A/F - O2 sensors and catalyst depend on correct operating temperatures, the vehicle should always be allowed to reach operating temperature and the sensor heater should always be checked The second test when checking A/F, O2 sensors and/or catalyst is to drive the system rich and lean with an Injector Volume or an A/F Control active test while observing the A/F and/or O2 sensor responses on the data list If the vehicle has two banks, compare the A/F and/or O2 sensor response of the suspected sensor to the opposite bank sensor in the same position If the responses vary, this may indicate a faulty A/F or O2 sensor Comparing the A/F and/ or O2 sensor response to a similar known good vehicle can also aid in diagnosis When the injection volume is commanded rich (+25%), an A/F sensor should drop to 3.0V or less, and an O2 sensor should raise to 0.5V or more (S1 within a few seconds, S2 within 20 seconds) When the injection volume is commanded lean (-12.5%), an A/F sensor should raise to 3.35V or more immediately, and an O2 sensor should drop to 0.4V or less (S1 within a few seconds, S2 within 20 seconds) If the S2 sensor responses fluctuate up and down frequently beyond the given thresholds, this may indicate a faulty catalytic converter The bank cycle time from lean to rich indicates the Oxygen Storage Capacity (OSC) of the catalyst The time starts when injector volume is commanded rich and A/F – O2S have the inverse reaction The lag time between sensor reactions indicate the OSC for that catalyst Technical Training 81 Technician Handbook 874 Engine Control Systems II O2 - A/F Sensor & Catalyst (OSC) Test 2: Injector Volume or A/F Control Active Test (cont’d) If either (one OR the other) of the exhaust sensors not fluctuate beyond the given thresholds, this may indicate a malfunctioning sensor, sensor heater, or sensor circuit If neither sensor fluctuates beyond the given thresholds, this may indicate an extremely rich or lean air/fuel ratio due to malfunctions in the fuel system, air induction system, exhaust leak, etc Typically, two exhaust sensors not malfunction at the same time In the illustration, an A/F Control active test was performed on a known good vehicle with a warm engine at idle in Park The first and third flags within the graph represents the injection volume commanded lean (-12.5%) The second flag represents the injection volume commanded rich (+25%) All of the A/F and O2 sensors responded correctly Refer to the Repair Manual for the correct diagnostic procedure and values NOTE The Injector Volume and A/F Control active tests place the ECM into Open Loop and the long-term fuel trim is used It may be helpful to clear the fuel trims by disconnecting the battery for several minutes before performing this test NOTE If the A/F or O2 sensor data list parameter displays a voltage reading well beyond the sensor’s detection range (i.e A/F = 5.8V, O2 = 1.2V), this may indicate an internally shorted heater circuit to signal circuit condition If a heater or circuit DTC is present, check the circuits using Repair Manual procedures 82 Technical Training Technician Handbook 874 Engine Control Systems II O2 and A/F Sensor Test 3: Fuel Enrichment and Fuel Cut Check the O2 Sensor monitor Test Results and Test Details prior to and after diagnosis and repair to confirm that the repair has been effective Because proper operation of the A/F and O2 sensors depends on correct sensor temperature, the heater should always be checked when testing the sensor The third test when checking A/F and/or O2 sensors is to drive the vehicle with the engine at operating temperature under heavy acceleration (fuel enrichment) and heavy deceleration (fuel cut) cycles while observing the A/ F and/or O2 sensor parameter readings on the data list During heavy acceleration, an A/F sensor (S1) should drop to 2.8V or less within a few seconds, and an O2 sensor (S1) should raise to 0.55V or more within a few seconds The O2 sensor in the S2 position reading should raise to 0.55V within 20 seconds during this condition During heavy deceleration (fuel cut), an A/F sensor (S1) should raise to 3.8V or more within a few seconds, and an O2 sensor (S1) should drop to 0.35V or less within a few seconds The O2 sensor in the S2 position reading should drop to 0.35V or less within 20 seconds during this condition If these output voltages are not met or exceeded during heavy accel and decel cycles, this may indicate a contaminated sensor Similarly, if the sensor output voltages not respond within the amount of time listed from idle to heavy accel and again from high engine RPM to heavy decel, this may also indicate a contaminated sensor If the output voltage of the A/F or O2 sensor remains at a fixed voltage during heavy accel and decel cycles, the sensor may have an open circuit This may also occur if the sensor heater has an open circuit If neither sensor fluctuates beyond the given thresholds, this may indicate an extremely rich or lean air/fuel ratio due to malfunctions in the fuel system, air induction system, etc If the output voltage of the A/F or O2 sensor remains high or low during heavy accel and decel cycles, the sensor may have a short circuit NOTE Technical Training Refer to the Repair Manual for the sensor response values and time limits for the vehicle you are working on 83 Technician Handbook 874 Engine Control Systems II Active Air/Fuel Ratio Control Some 2003 and most later model vehicles perform Active Air/Fuel Ratio Control to detect malfunctions in the A/F, O2 sensor(s) and deterioration in the three-way catalytic converter(s) Active Air/Fuel Ratio Control is performed while driving with a warm engine and can set a variety of DTCs During Active Air/Fuel Ratio Control, the air/fuel ratio is forced lean and rich by the ECM If the ECM detects a malfunction in the A/F sensor, three-way catalytic converter, or O2 sensor, a DTC will be set To determine accurate air fuel (A/F) control the ECM will conduct a series of process/steps upon start up to verify system integrity Active Air/Fuel Ratio Control is a three stage process Stage 1: A/F Learning A/F Learned Control is another name for Long Term Fuel Trim Before the ECM can enter active A/F Control the feedback system needs to be stabilized at Stoichiometric Based on the feedback from the A/F sensor the ECM will determine correction values (Short FT) to stabilize A/F ratio In general, a guard value of +/- 20% is provided for an A/F feedback value, and A/F control is performed within this range A/F Learned Control or Long FT compensates for A/F deterioration for specific engine load conditions Conditions for Learning: 1.  System is normal (No DTC detected etc) 2.  The engine is warmed up (to a coolant temperature of 150 F or more) 3.  The integrated value of MAF from the engine starting is not lower than a specified value 84 Technical Training Technician Handbook 874 Engine Control Systems II Stage 1: A/F Learning (cont’d) 4.  The feedback cycle (rich, lean) is stable 5.  The average of Short FT is not within +- % In such conditions, Short FT reaches a constant corrections value for that load range That value then becomes a Long Term FT value and learning takes place This learning cycle will continue until fuel control is stable and correction value has not reached the guard value, which is generally +- 40% Stage 2: A/F Sensor Output Judgment NOTE Stage 3: Active A/F Control Technical Training The A/F sensor output judgment is made after the end of the A/F learning and before the start of active A/F control The ECM forcible shifts Target A/F, rich and lean and observes whether A/F sensor output follows the variances This allows control target values to be confirmed based on the target A/F, so it can be determined whether control is taking place or not A/F sensor output judgment is carried out automatically and forcibly during driving Therefore, in order to avoid confusing it with “A/F variance” when checking the data list due to malfunction, you should check the values of target A/F to determine whether the data is the result of control or vehicle malfunction •  Target value for rich: 0.965 approx 14.0 A/F ratio •  Target value for lean: 1.035 approx 15.0 A/F ratio After the A/F sensor is judged to be normal, the 3-way catalyst and O2 sensor judgment is made During Active A/F Control the following test are preformed: •  The 3-way catalytic converter and the O2 sensor (S2) system are checked once in a trip to judge whether they are normal of not •  A switch between “lean” and “rich” is repeated forcibly, and the time it takes for O2 sensor (s2) output to switch as well as its timing are checked to give a judgment •  The “timing of O2 sensor (S2) output switching between lean and rich (the point at which a switch of 0V to 1V or vice versa takes place) is checked three to five times in one round of judgment If the result is judged to be normal, then the control during that trip ends •  Control is suspended if the enable conditions for control are violated during driving If the enable conditions are reestablished, the control is resumed What this means is that a diagnosis process may fail to be completed depending on driving conditions 85 Technician Handbook 874 Engine Control Systems II A/F sensor output judgment is carried out automatically and forcibly during driving Therefore, in order to avoid confusion with possible “A/F variance”, you should check the value of target A/F to determine whether the data is the result of active control or vehicle malfunction Target A/F values usually fluctuate between 0.956 (Target Rich approx 14.0:1) to 1.026 (Target Lean approx 15.0:1) The illustration above represents a OBD Trip where all monitors were able to run and complete From the large view it is difficult to pin point where A/F sensor judgment is taking place When zoomed in it is possible to see where the judgment was taking place The key point to remember about Active A/F Control is Readiness Monitor Drive Patterns If the enabling conditions are met and drive patterns are followed The judgment will be made automatically and associated DTCs will set When monitor pattern completes and no DTCs are found the vehicle is judged to be normal and monitors will state Pass If any pending DTCs set access monitor results to determine which monitor test failed NOTE 86 Always refer to the Repair Manual for vehicle specific Readiness Monitor Drive Patterns Technical Training Technician Handbook 874 Engine Control Systems II Active AF Control Capture-Good Catalyst Response Technical Training Screenshot represents a good A/F sensor & catalyst judgment Notice how the ECM is commanding target A/F ratio rich and lean This verifies A/F response as well as O2 sensor cycle for catalyst judgment 87 Technician Handbook 874 Engine Control Systems II Active AF Control Capture-Failed Catalyst Response 88 The screenshot above shows how Active A/F control determines a bad catalyst Technical Training Technician Handbook 874 Engine Control Systems II Technical Training 89 Technician Handbook 874 Engine Control Systems II 90 Technical Training ... sensor Technical Training 71 Technician Handbook 874 Engine Control Systems II 72 Technical Training Technician Handbook 874 Engine Control Systems II O2 Sensor Monitors Technical Training The O2... Technical Training Technician Handbook 874 Engine Control Systems II Technical Training 89 Technician Handbook 874 Engine Control Systems II 90 Technical Training ... ignition ON, engine OFF and engine ON to a similar known good vehicle at the same temperature can also aid in diagnosis Technical Training Technician Handbook 874 Engine Control Systems II A/F