Heated Oxygen Sensors (HO2S) - From 99MY
The number of heated oxygen (HO2S) sensors fitted to a vehicle is dependent on the particular market requirements:
- 4 - HO2S sensors (NAS vehicles)
- 2 - HO2S (UK, European, Australia & Japan vehicles)
- 0 - HO2S (Gulf & ROW vehicles)
The HO2S sensors monitor the level of oxygen in the exhaust gases and the resulting data is used by the ECM to control the air:fuel mixture to provide the most efficient mix under all operating conditions. By positioning a sensor in the stream of exhaust gases from each bank of cylinders of the V8 engine enables the ECM to control the fuelling on each bank independently. This allows the ECM to provide more accurate control of the air:fuel ratio and monitor catalytic converter efficiency.
Two upstream sensors are utilized in markets where closed loop fuelling is the only mandatory requirement. For markets where closed loop fuelling control is not mandatory, HO2S sensors are not included.
NAS vehicles utilize four HO2S sensors, one upstream of each catalyst and one downstream of each catalyst. This arrangement is used to monitor catalytic converter efficiency and so determine when a catalyst is no longer working effectively. Obtaining catalytic converter efficiency data is a mandatory requirement of the ECM OBD strategy. The downstream sensors also provide for long term fuelling adoptions.
The basic closed control loop comprises the engine (controlled system), the heated oxygen sensors (measuring elements) and the engine management ECM (control) and the injectors and ignition (actuators). Although other factors also influence the calculations of the ECM, such as air flow, air intake temperature and throttle position. Additionally, special driving conditions are compensated for such as starting, acceleration and full load.
From cold start the ECM runs an open loop strategy, which is kept in place until the sensor's working temperature has been reached.
The heated oxygen sensors age with mileage, which will cause an increase in their response time for switching from rich to lean and lean to rich. The increase in response time influences the closed loop control and leads to progressively increased emissions. If the response rate is diagnosed to be exceeding a preset threshold, an error code will be stored in the ECM and the MIL warning lamp will be illuminated (NAS only).
The heated oxygen sensor is protected by an outer tube with a restricted flow opening to prevent the sensors ceramics from being cooled by low temperature exhaust gases at start up. The pre-catalytic sensors are identified by three slots in the protective tube, whereas the post-catalytic sensors have four square indentations and a hole in the end of the protective tube (NAS only).
NOTE: The maximum working temperature of the tip of the HO2S sensor is 930° C (1706° F); temperatures higher than this will damage the sensor.
The heater elements are controlled by a PWM signal from the ECM. The heater elements are operated immediately following engine start and also during low load conditions when the temperature of the exhaust gases is insufficient to maintain the required sensor temperatures. The heater element warms the sensor's ceramic layer from the inside so that the sensor is hot enough for operation. After start up, the sensors are ready for closed loop control within about 20 to 30 seconds.
If the heater element fails, the ECM will not allow closed loop fuelling to be implemented until the sensor has achieved the required temperature. A diagnostic routine is utilized to measure both sensor heater current and the heater supply voltage, so its resistance can be calculated. The function is active once per drive cycle, as long as the heater has been switched on for a pre-defined period and the current has stabilized. The PWM duty cycle is carefully controlled to prevent thermal shock to cold sensors.
The pre-catalytic and post-catalytic converters are not interchangeable, and although it is possible to mount them in transposed positions, their harness connections are of different gender and colour:
- Upstream sensors have orange connectors.
- Downstream sensors have grey connectors.
It is important not to confuse the sensor signal pins; the signal pins are gold plated, whilst the heater supply pins are tinned, mixing them up will cause contamination and effect system performance with time.
NOTE: Sensor voltage is most easily monitored using 'Testbook".
If a heated oxygen sensor should fail, the following symptoms may be observed:
- Default to open loop fuelling on the catalyst bank with the failed sensor.
- If sensor get crossed, the engine will run normally after the initial start, but then become progressively unstable. One bank will clamp at the maximum rich level, and the other bank will clamp at maximum lean. The system will then revert to open loop fuelling.
- High CO reading
- Excess emissions
- Strong smell of hydrogen sulphide (H2S) until the ECM defaults to open loop fuelling
- MIL lamp illuminated (NAS only)
A heated oxygen sensor failure is likely to occur for the following reasons:
- Damaged or incorrectly fitted sensor
- Sensor open circuit or disconnected
- Short circuit to vehicle supply or ground
- Stoichiometric ratio outside the correct operating band
- Contamination from leaded fuel or other sources
- Change in sensor characteristics - Chemical Shift Down (CSD)
- Sensors from LH and RH banks crossed
- Air leak into exhaust system (cracked pipe/weld or loose fixings)
Diagnosis of electrical faults is continually monitored by the ECM in both the upstream sensors and downstream sensors (NAS only). The sensor signal is checked against stored minimum and maximum threshold values equating to short and open circuit conditions.
If an HO2S sensor should fail, the following fault codes will be generated by the ECM diagnostics, which can be retrieved by Testbook:
- P1129 - Front heated oxygen sensors transposed
Upstream sensor LH bank - electrical
- P0130 - Stoichiometric ratio outside operating band
- P0132 - Short circuit to battery supply
- P0134 - Open circuit
Downstream sensor LH bank - electrical
- P0136 - Stoichiometric ratio outside operating band
- P0137 - Short circuit to battery supply
- P0138 - Short circuit to ground or chemical shift down
- P0140 - Open circuit
Upstream sensor RH bank - electrical
- P0150 - Stoichiometric ratio outside operating band
- P0152 - Short circuit to battery supply
- P0154 - Open circuit
Downstream sensor RH bank - electrical
- P0156 - Stoichiometric ratio outside operating band
- P0157 - Short circuit to battery supply
- P0158 - Short circuit to ground or chemical shift down
- P0160 - Open circuit
Upstream sensors aged
- P0133 - Upstream sensor aged - Period time too short LH bank
- P0133 - Upstream sensor aged - Period time too long LH bank
- P0153 - Upstream sensor aged - Period time too short RH bank
- P0153 - Upstream sensor aged - Period time too long RH bank
- P1170 - Upstream sensor aged - ATV adaption too lean LH bank
- P1170 - Upstream sensor aged - ATV adaption too rich LH bank
- P1173 - Upstream sensor aged - ATV adaption too lean RH bank
- P1173 - Upstream sensor aged - ATV adaption too rich RH bank
Sensor Heater faults
- P0135 - Upstream heater LH bank - Short circuit
- P0135 - Upstream heater LH bank - Open circuit
- P0141 - Downstream heater LH bank - Short circuit
- P0141 - Downstream heater LH bank - Open circuit
- P0155 - Upstream heater RH bank - Short circuit
- P0155 - Upstream heater RH bank - Open circuit
- P0161 - Downstream heater LH bank - Short circuit
- P0161 - Downstream heater LH bank - Open circuit
A diagnostic routine is used to measure both sensor heater current and the heater supply voltage so its resistance can be calculated. The function is active once per drive cycle as long as the heater has been switched on for a pre-defined period and the current has stabilized. The PWM duty cycle is carefully controlled to prevent thermal shock to cold sensors.
On NAS vehicles, the catalysts are monitored to determine emission pollutant conversion efficiency; the following fault codes will be generated by the ECM diagnostics, which can be retrieved by Testbook:
- P0420 - Catalyst efficiency deteriorated LH bank
- P0430 - Catalyst efficiency deteriorated RH bank
See EMISSION CONTROL, Description and operation.