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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 2 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 NOTE NOTE 64 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. 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 .4 volts. AF- (2.9 volts contant) + electromotive force .4 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 2 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 When O2 sensor voltage is compared to A/F sensor output it is easy to see the accuracy differences over the entire air fuel ratio range. It is important to remember that voltage readings for the A/F sensor are calculated by the ECM and cannot be measured at the 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 – A/F sensors, heaters, and catalyst are monitored by three separate readiness monitors: the O2 Sensor, O2 Sensor Heater and Catalyst Efficiency monitors. All three monitors require different readiness monitor drive patterns. Refer to the Repair Manual for the specific readiness monitor drive patterns and enabling conditions. 73 Technician Handbook 874 Engine Control Systems II O2 Sensor Monitor Monitor Methods The O2 Sensor monitor checks for A/F and O2 sensor circuit malfunctions, slow response rate, no activity, and/or high or low voltage. This monitor also checks for the sensor signal stuck lean or rich. The O2 Sensor monitor changed from a passive to an active system with some 2003 and later models. •  Prior to 2003, and some 2003 models: The O2 Sensor monitor checks for sensor circuit malfunctions, for slow response rate, and for a malfunction of the sensor’s heater circuit. Each condition sets a DTC. If the ECM detects varying fuel injection volumes, it takes advantage of the condition to measure signal response (fast sensor response indicates a good sensor). •  Some 2003 and most later models: The Active Air/Fuel Ratio Control monitor forces the air/fuel ratio lean and rich to check for deterioration in catalytic converter performance as well as A/F sensor and/or O2 sensor malfunctions. In addition to active testing, the ECM monitors sensor output during certain engine operating conditions (such as closed throttle deceleration) and can set DTCs based on abnormal sensor readings. 74 Technical Training Technician Handbook 874 Engine Control Systems II A/F and O2 Sensor DTCs •  O2 (A/F) Sensor Circuit Faults: Low Voltage, High Voltage, Malfunction •  O2 (A/F) Sensor Signal Faults: Stuck Rich, Stuck Lean, Slow Response, No Activity •  O2 (A/F) Sensor Pumping Current Circuit Faults: Open, Low/High •  O2 (A/F) Sensor Reference Ground Circuit: Low/High O2 Sensor Heater Monitor Heater DTCs The O2 Sensor Heater monitor checks for A/F and O2 sensor heater circuit malfunctions, and high or low voltage. •  Heater Control Circuit: High, Low, and Malfunction •  Circuit Faults: Low Voltage, High Voltage, Malfunction NOTE Technical Training Depending on vehicle year, make, and model, the O2 Sensor and O2 Sensor Heater monitors may not check for all of the malfunctions listed in the illustration. Refer to the Repair Manual for specific DTC information. 75 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 3 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 1 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 1 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