Automotive Oxygen Sensor Testing
By Mandy Concepcion
The O2 sensor measures the oxygen content of the exhaust. The O2 sensor’s sensing ability comes about by producing a small voltage proportionate to the exhaust oxygen content. In other words, if the oxygen content is low it produces a high voltage (0.90 Volts - Rich mixture) and if the oxygen content is high it produces a low voltage (0.10 Volts - Lean mixture). Although theoretically the O2 sensor should cycle between 0.00 volts and 1.00 volts, in reality it cycles between 0.10 volts and 0.90 volts.
A GM O2 sensor signal stuck at 450 mV is an indication of an open O2 sensor circuit (signal wire) or faulty O2 signal ground. The 450 mV value (GM) is called a bias voltage and it is not the same for all manufacturers. Some manufacturers employ a dedicated O2 sensor ground. Such a ground lead is attached to the engine block or chassis and feeds an ECM O2 ground pin only. The O2 circuit is then grounded through the inside of the ECM electronic board by this ground wire. A loss of this ground would also put the O2 sensor signal at around 450 mV, which also makes it look like an open circuit. The same holds true for Chrysler, but these use a different O2 bias voltage, which is usually 2.00 to 4.00 volts
1) A few key issues are very important in the analysis of O2 sensor signals. 2) An O2 sensor will cycle between 0.10 to 0.90 or almost 1 volt. 3) An O2 sensor has to reach the 0.8x Volts amplitude mark while at full operation. 4) An O2 sensor also has to reach the 0.1x Volts amplitude mark while at full operation. 5) Full operation means the engine is fully warmed up, O2 sensor above the 600 deg. F. operating temperature, and no fuel or mechanical problems present. 6) The O2 sensor must cycle at least once per second, which would show 3 cross counts on the scan tool PID. 7) Silicone is the leading cause of O2 contamination. 8) It is easier for an O2 sensor to go from rich to lean than vise-versa. 9) O2 sensors tend to fail on rich bias. In other words, they tend to shift their cycling to the upper side or rich side of the voltage scale. 10) Contrary to what many people think, an O2 sensor WILL NOT cycle by itself. The O2 sensor cycle is a direct result of the ECM response to the changes in the mixture. 11) Any time the O2 cycles and crosses the 0.450 volts mark, the system is in CLOSE-LOOP. 12) Even though an O2 sensor is cycling and crossing 0.450 volts (ECM in close loop) it DOES NOT mean that it is working properly. 13) O2 sensor operation is extremely important not only to keep HC & CO emissions low but also to the NOx as well. 14) Proper O2 sensor cycling will determine the catalytic converter’s efficiency. The catalytic converter needs the O2 sensor cycling at its proper amplitude and frequency for it to function at its maximum efficiency. 15) An O2 sensor with a high voltage reading does not necessarily mean that the mixture is rich or high in fuel content. An EGR valve problem will send the O2 signal high as well.
A big misconception among technicians trying to understand O2 sensors is that they cycle by themselves. The O2 sensor just reads oxygen content in the exhaust, THAT’S IT. Excess oxygen in the form of regular ambient air will send the O2 sensor voltage signal low (under 0.450 volts) and lack of it will send the voltage signal high (over 0.450 volts). A stuck open EGR valve will create a lack of oxygen in the exhaust, since the re-circulating exhaust has all its oxygen already burnt . The ECM sometimes uses the O2 sensor to check for proper EGR operation and sets a code if necessary. So, be aware of the fact that a vehicle might be running lean because the ECM sees a rich O2 signal due to a defective (stuck open) EGR valve. Since the ECM sees a rich signal, it will try to correct with a lean command and try to lower the O2 sensor’s high voltage signal.
CONDITION THAT AFFECT OPERATION
NOTE: WHEN PERFORMING O2 SENSOR CHECKS, IT IS IMPORTANT TO TAKE MEASUREMENTS AT IDLE AND 2000 RPM. BE AWARE THAT O2 SENSOR PRE-CONDITIONING IS IMPORTANT, EVEN ON THE NEWER STYLED HEATED O2 SENSORS. PRE-CONDITION THE O2 SENSOR BY RAISING THE ENGINE SPEED TO 2000 RPM FOR ABOUT 15 SECONDS OR SO. THE O2 SENSOR HAS TO BE ABOVE 600 º F. TO BE ABLE TO OPERATE PROPERLY. LONG PERIODS OF IDLE TIME CAN RENDER A NON-HEATED OR OLDER O2 SENSOR TOO COLD FOR IT TO FUNCTION AT ALL. AT THE SAME TIME, DO NOT TRY TO FORCE A HEATED O2 SENSOR INTO OPERATION. AN O2 SENSOR WITH A FAULTY HEATER WILL GO INTO CLOSED-LOOP AFTER A GOOD WARM-UP SESSION.
After an engine has ran through its warm up period (O2 sensor has no effect on engine operation while the engine is cold), the ECM then looks for the O2 value. The 0.450 volts mark is considered almost universally as the midway point or crossover point for O2 sensor operation. If the signal is on the rich side (above 0.45 volts), then the ECM will answer with a lean command (reducing injector pulsation), or if the signal is on the lean side (below 0.45 volts) then the ECM will answer with a rich command (increasing injector pulsation). The amount of injector pulse correction is proportional to the voltage seen by the ECM at the O2 sensor signal wire. The higher the voltage the more the ECM reduces on-time to the injector. The lower the voltage the more the ECM increases the injector on-time. The ECM is constantly doing exactly just that, slightly increasing and decreasing injector pulsation. The constant adjustment is what gives the O2 sensor signal the switching appearance (sine wave) on the scope screen.
NOTE: The ECM’s fuel pulse corrections performed constantly to the injector signal is called SHORT TERM FUEL TRIM ( GM called it INTEGRATOR ) and LONG TERM FUEL TRIM ( GM called it BLOCK LEARN ) on the scanner. FUEL TRIMS is the system’s deviation of the BASE-INJECTION pulse. Analyzing LTFT and STFT is a great way to know a particular vehicle’s fuel consumption trend or how well that vehicle has been performing with regards to fuel control. STFT and LTFT is the first thing to look for when assessing fuel control problems.
The fact that the O2 sensor signal is switching rich-lean-rich-lean also reveals that the ECM is controlling the injector pulsation and therefore that the system is in close loop mode. An ECM in full control (O2 sensor cycling) is said to be in close loop because of the close-circuit action of O2 sensor-to ECM-to injector pulse control then to O2 sensor and back to the ECM. The ECM must be in control at all times except during warm up, WOT, power enrichment, and deceleration mode.
The O2 sensor not only has to cycle, it also has to cycle fast enough (proper frequency) and wide enough (proper amplitude). At least one cycle per second ( 1 Hz ) must be seen at the signal wire in order for the O2 to be considered good (not lazy). A one cycle per second will make the scope trace go across the 0.450 volts mark approximately 3 times, which the ECM recognizes as 3 cross counts. A slow O2 sensor will have a damaging effect on the catalytic converter and release excessive amounts of emissions to the atmosphere.
A cycle are the complete rich and lean crests of the O2 sensor signal, while crossing the 0.45 voltage point. Proper amplitude refers to the O2 sensor’s ability to reach full rich ( 0.90 volts ) and full lean ( 0.10 volts ) when cycling. The higher the voltage seen at the O2 signal line the more the ECM reduces pulsation to the injectors. The lower the voltage seen at the O2 signal line the more the ECM increases injector pulsation. This is the reason why an O2 sensor that is not reading the mixture properly, at full amplitude and frequency, will actually misguide the ECM into a wrong fuel control pattern. Once the O2 sensor has reached its correct temperature of 600 º F, look for an O2 signal cycle with the correct amplitude and frequency and it will surely indicate a perfectly operating O2 sensor.
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NOTE: On early OBD II systems, the post catalytic converter O2 sensor has no effect on fuel control. The post catalytic O2 sensor was originally responsible for only monitoring catalytic converter efficiency. On most systems, the post converter O2 sensor signal should never mimic or follow the pre-cat O2 signal. That would indicate a defective or low oxygen storage capability at the converter . On early OBD II systems, the post-cat O2 sensor should show little or no voltage fluctuations on a scope waveform, since all the mixture fluctuations are being absorbed by the catalytic converter.
Stating around model year 1999, a new type of converter came on the market, called “Low Oxygen Storage Converter” or LOC. With an LOC, the pre and post O2 sensors cycle at the same rate. These converters are tested by measuring the lag-time between the two signals. A further development of this system is that the post converter signal is also used for A/F correction, but to a less extent.
These simple steps should be followed whenever testing O2 sensors.
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