The rear heated oxygen sensor is mounted on the rear side of the Catalytic Converter (warm-up catalytic converter) or in the rear exhaust pipe, which is able to detect catalyst efficiency. The rear heated oxygen sensor (HO2S) produces a voltage between 0V and 1V. This rear heated oxygen sensor is used to estimate the oxygen storage capability. If a catalyst has good conversion properties, the oxygen fluctuations are smoothed by the oxygen storage capacity of the catalyst. If the conversion provided by the catalyst is low due to aging, poisoning or misfiring, then the oxygen fluctuations are similar to signals from the front oxygen sensor.
Checking the delayed time between the front sensor response and the rear sensor response to fuel shift under detecting condition, if the fuel shift time is higher than 25sec, PCM determines a fault and sets DTC P0139. MIL(Malfunction Indication Lamp) turns on when the malfunction lasts till consecutive 2 driving cycle.
Item | Detecting Condition | Possible cause |
DTC Strategy | ● Determines if Rear O2 Sensor is acceptable for Idle Catalyst Monitor use | ● Poor connection ● Faulty HO2S ● Faulty PCM |
Enable Conditions | ● Same conditions as idle catalyst monitoring | |
Threshold value | ● Response time of near oxygen sensor by shifting airfuel ratio from lean to rich and vice versa. ≥ 25 sec. | |
Diagnosis Time | - | |
MIL On Condition | ● 2 driving cycles |
Air/fuel mixture | Voltage(V) |
Rich | 0.75 ~ 1V |
Lean | 0 ~ 0.12V |
