Heated Oxygen Sensor
| ITEM | DESCRIPTION |
|---|---|
| 1 | Heated Oxygen Sensor (HO2S) - Pre catalyst (quantity 2) |
| 2 | HO2S - Mid catalyst (quantity 2) |
| 3 | HO2S - Post catalyst (quantity 2) |
The Heated Oxygen Sensors (HO2S) allow the Powertrain Control Module (PCM) to measure the oxygen content of the exhaust gases. This is for closed loop control of the fuel and air mixture and for catalytic converter monitoring.
A pre-catalyst HO2S is located in the outlet of each exhaust manifold. This sensor enables independent control of the fuel and air mixture for each cylinder bank. A mid catalyst HO2S is located in a central position on the side of each catalytic converter. A post catalyst HO2S is located in each catalytic converter outlet pipe in the exhaust system. The sensors enable the operation of the catalytic converter to be optimized and monitored.
HO2S need to operate at high temperatures in order to function correctly. To achieve the high temperatures required, the sensors have heater elements. The PCM controls the heater elements by a Pulse Width Modulation (PWM) signal.
The heater elements are operated after each engine start:
- When it has been calculated that there is no moisture in the exhaust.
- Also during low load conditions when the temperature of the exhaust gases is insufficient to maintain the required sensor temperature.
The time period for operation of the heater elements is determined by the temperature of the post catalyst HO2S. The PWM duty cycle is carefully controlled to prevent thermal shock to cold sensors. A non-functioning heater delays the sensor's readiness for closed loop control and increases emissions.
The pre catalyst HO2S produce a constant voltage, with a variable current that is proportional to the lambda ratio. The mid and post catalyst HO2S produce an output voltage dependant on the ratio of the exhaust gas oxygen to the ambient oxygen.
Diagnosis of electrical faults only begin when the sensors have reached operational temperature and become active. This is achieved by checking the signal against maximum and minimum threshold, for open and short circuit conditions.
If an HO2S fails, the engine performance may be reduced, depending on which sensor it is. The Malfunction Indicator Lamp (MIL) will always be displayed if an HO2S fails.