Rear Ho2S Heaters (2018)

The HO2S heaters are monitored for proper voltage and current. A HO2S heater voltage fault (open, shorted to ground, or shorted to battery) is determined by turning the heater on and off and looking for corresponding voltage change in the heater output driver circuit in the PCM.

Beginning 2014MY, some applications have rear HO2S that uses impedance feedback heater controller to control impedance to a corresponding element temperature. The HO2S impedance feedback heater controller keeps the HO2S heater at constant impedance (Bosch LSF-X4: 220 ohms, NTK S2.01: 150 ohms), which corresponds to a temperature (Bosch LSF-X4: 780 deg C, NTK S2.01: 650 deg C). The impedance of the HO2S decreases as the sensor temperature increases. After a cold start, the HO2S heater ramps up to the maximum duty cycle to heat the sensor. After a few seconds, the measured impedance will start to decrease and when the target value is crossed, the heater goes into closed loop heater control to maintain the sensor at the target impedance.

The "HO2S Heater Temperature Control Monitor" tracks the time at the maximum duty cycle during the open loop sensor warm up phase. If the measured impedance does not come down to the target value to allow the system to transition from open loop heater control to closed loop heater control within a specified time, then a P0036/P0056 fault code is set. This monitor also sets a malfunction when the closed loop heater controller reaches a maximum or minimum duty cycle for a period of time indicating that the controller is no longer able to maintain the target impedance. However, if the exhaust temperature is high enough that the sensor will be below the target impedance even with no heat, then this monitor is disabled.

A separate current-monitoring circuit monitors heater current once per driving cycle. This monitor normally runs after the HO2S heaters have been on for some time and inferred to have been adequately warm. However, on applications that use HO2S impedance feedback heater controller, it can also run intrusively. When the HO2S impedance indicates cold, but the heater is inferred to have been adequately warm, the current monitor is forced to run intrusively prior to the completion of the heater temperature control monitor. If the current measurement falls below or above a calibratable threshold, the heater is assumed to be degraded or malfunctioning.

Beginning 2012MY, some PCMs do not have a separate heater current-monitoring circuit (without shunt resistors that can directly measure the current through the HEGO heaters). For applications that do not have a heater current-monitoring circuit and also do not use HO2S impedance feedback heater controller, the sensor heater performance is monitored by the "HO2S Impedance Monitor". The HO2S impedance monitor measures the HO2S internal impedance, validates the measurement, and then compares the validated internal impedance to an internal impedance threshold. If the validated internal impedance exceeds the threshold, then the monitor fault counter increments once. If the fault counter exceeds the total number of valid internal impedance measurements required, a HO2S heater control circuit range/performance malfunction (P00D2/P00D4) will be set.

The HO2S impedance monitor runs once per trip; however, it can also be forced to run intrusively. When the heater is inferred to have been adequately warm, but the HO2S sensor is suspected to be cold because the HO2S voltage falls inside the suspected open HO2S circuit voltage fault band or inside the suspected HO2S circuit shorted to ground voltage fault band, a HEGO sensor circuit or HEGO heater malfunction is suspected. To differentiate HO2S signal circuit failures from a degraded/malfunctioning heater or normal FAOS control, the HO2S impedance monitor is forced to run intrusively after the heater voltage test and the HO2S open/short to ground circuit diagnostics had ran and indicated no malfunction.

Any corrosion in the harness wiring, connector, or increase in the sensor heater element resistance will result in an overall increase in the heater circuit resistance, causing the HO2S impedance to increase. The impedance is dependent on the HO2S element temperature and the voltage at the connector. As the HO2S element temperature increases, the impedance decreases. Furthermore, as the voltage at the connector increases, the sensor impedance decreases. Hence, the impedance threshold for the HO2S heater impedance monitor is a function of the inferred HO2S element temperature and the voltage at the connector.

On the other hand, applications that use HO2S impedance feedback heater controller and also do not have a separate heater current-monitoring circuit, a degraded or malfunctioning HO2S heater is detected by the "HO2S Heater Temperature Control Monitor." This monitor would call the malfunction after the heater voltage test and HO2S circuit diagnostics had run first and indicated no malfunction.

Courtesy of