Heated Oxygen Sensors
| ITEM | DESCRIPTION |
|---|---|
| 1 | Heated Oxygen Sensor (HO2S) - Post catalyst - North American Specification (NAS) vehicles only |
| 2 | HO2S - Pre catalyst |
| 3 | HO2S - Mid catalyst |
The Heated Oxygen Sensors (HO2S) allow the Powertrain Control Module (PCM):
- To measure the oxygen content of the exhaust gases
- To control the fuel/air mixture
- For long and short fuel trim calculation
- For catalytic converter monitoring.
The pre catalyst HO2S is installed in the outlet of exhaust manifold, which enables independent control of the fuel/air mixture for each combustion chamber. The mid catalyst HO2S is installed in a central position on the side of the catalytic converter. The post catalyst HO2S is installed in the catalytic converter outlet pipe in the exhaust system. These enable the performance of the catalytic converter to be optimized and monitored.
The HO2S needs to operate at high temperatures in order to function correctly. To achieve the required high temperatures, the sensors are fitted with heater elements that are controlled by a Pulse Width Modulation (PWM) signal from the PCM. The heater elements are operated after each engine start, once it has been calculated that there is no moisture in the exhaust. The calculation causes a delay between zero and ten minutes.
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 HO2Ss. A nonfunctioning heater delays the sensors 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 dependent on the ratio of the exhaust gas oxygen to the ambient oxygen.
The HO2S age 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 sensors.
Diagnosis of electrical faults is continually monitored in pre, mid and post catalyst HO2Ss. This is achieved by checking the signal against maximum and minimum threshold, for open and short circuit conditions.
In the event of failure a Diagnostic Trouble Code (DTC) is recorded within the PCM and the Malfunction Indicator Lamp (MIL) is illuminated.
If a HO2S fails:
- The PCM defaults to open loop fueling for the cylinder bank.
- The Carbon Monoxide (CO) and emissions content of the exhaust gases may increase.
- Reduced engine refinement and performance.