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.
The PCM monitors rich-lean switching time of rear heated oxygen sensor (HO2S) after fuel cut-off to validate dynamic behavior of rear heated oxygen sensor (HO2S). After detection of fuel cut-off engine operating state, the PCM measures rich-lean switching time of the rear heated oxygen sensor (HO2S) signal and compares it to the predetermined limit value. DTC P0139 is set when the switching time is bigger than the limit value.
Item
Detecting Condition
Possible Cause
DTC Strategy
Slow response(Switching time check at entry in fuel cut off)
Leak in intake or exhaust system
Faulty fuel system.
Front and rear HO2S connections reversed.
Poor connection or damaged harness
HO2S contamination
Enable Conditions
Coolant temperature >73?(163?)
Downstream O2 sensor readiness detected
Downstream O2 sensor pre-heating phase finished
400?(752?)< Catalyst temperature model
0.6V< Downstream O2 sensor signal at entry into fuel cut-off
No relevant failure
11V < Battery voltage < 16V
Threshold Value
Average ratio between measured and maximum allowed rich to lean switching time entering fuel cut-off >1
Diagnostic Time
4 valid fuel cut-off phases
Mil On Condition
2 Drive Cycles
? Sector A : Signal Normal
? Sector B : Signal Fluctuation
? Sector C : Signal Recovery