Test Lambda closed loop control
Content/ topic 2. Test Lambda closed loop control
The O2 or Oxygen sensor works like a tiny voltage generator. It actually produces a voltage in the theoretical range of 0.01 to 0.98 volts. It does this depending on the Oxygen content of the exhaust. This signal is a major input to the ECM, which it uses to control the air-fuel mixture and emissions.
Theorxy of Operation
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.
- In many modern cars the O2 sensor has been replaced by the AFR or Wide-Band sensor. But, the rear or after-cat O2 sensor is still the same old fashioned O2 sensor.
A few key issues are very important in the analysis of O2 sensor signals.
- An O2 sensor will cycle between 0.10 to 0.90 or almost 1 volt.
- An O2 sensor has to reach the 0.8x Volts amplitude mark while at full operation.
- An O2 sensor also has to reach the 0.1x Volts amplitude mark while at full operation.
- (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. Also most rear O2 sensor will not cycle as wide, but during testing you have to goose the throttle and do a few WOT events to prove your O2 sensor voltage swing.)
- The front O2 sensor must cycle at least once per second, which would show 3 cross counts on the scan tool PID.
- Silicone is the leading cause of O2 contamination.
- It is easier for an O2 sensor to go from rich to lean than vise-versa.
- 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.
- There is a small hole at the body of the O2 sensor, which allows it sample the outside reference Oxygen.
- 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.
- When the O2 cycles and crosses the 0.450 volts mark every second, the system is in CLOSE-LOOP.
- 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.
- O2 sensor operation is extremely important not only to keep HC & CO emissions low but also to reduce NOx as well.
- 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.
- 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.
- The 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 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 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. Remember, this bias voltage circuit is very low current.