Heated Oxygen Sensor
The Heated Oxygen Sensor (HO2S) is located in a threaded boss in the inlet to the catalytic converter and Diesel Particulate Filter (DPF) assembly.
The HO2S allows the Powertrain Control Module (PCM) measure the oxygen content of the exhaust gases:
- For closed loop control of the fuel/air mixture
- For catalytic converter monitoring
- For DPF assembly monitoring.
The HO2S must operate at high temperatures in order to function correctly. To achieve the high temperatures required, the sensor is equipped with a heater element. The heater element is controlled by a Pulse Width Modulation (PWM) signal from the PCM.
The heater element is operated after each engine start, once it has been calculated that there is no moisture:
- In the inlet to the catalytic converter
- In the DPF assembly (between 0 and 10 minutes delay).
The heater element is also operated during low load conditions when the temperature of the exhaust gases is insufficient to maintain the required sensor temperature. The PWM duty cycle is carefully controlled to prevent thermal shock to a cold sensor. A non-functioning heater delays the sensor's readiness for closed loop control and increases emissions.
The HO2S produces a constant voltage, with a variable current that is proportional to the lambda ratio.
The HO2S deteriorate with mileage, increasing their response time to switch from rich to lean and lean to rich. This increase in response time influences the PCM closed loop control and leads to progressively increased emissions. Measuring the period of rich to lean and lean to rich switching monitors the response rate of the pre catalyst HO2S.
Diagnosis of electrical faults is continually monitored. This is achieved by checking the signal against maximum and minimum threshold, for open and short circuit conditions.
If the HO2S fails:
- The PCM defaults to open loop fueling
- The Carbon Monoxide (CO) and emissions content of the exhaust gases may increase.
The engine will show reduced refinement and performance if the HO2S fails.