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.
Due to possible oxygen sensor defects (e.g. reference air poisoning) or faults in the injection system (e.g. leaking fuel injector), the rear oxygen sensor may not provide the expected lean or rich signal level during fuel cut-off or full load condition. Hence, the oxygen sensor signal is checked for plausibility during this engine operating states.There are 2 cases which DTC P0140 sets.
(I) Signal monitoring during fuel cut-off: The PCM monitors rear O2 sensor signal level during fuel cut-off which normally shows near 0V and sets DTC P0140 when signal level is too high.
(II) Signal monitoring after fuel cut-off: The PCM monitors rear O2 sensor signal level after leaving fuel cut-off and sets DTC P0140 when signal remains at or below 0.6V.
Item
Detecting Condition
Possible Cause
DTC
Strategy
Case 1
Signal plausability with rich mixture (during catalyst purge after fuel cut-off)
Poor connection or damaged harness
HO2S contamination
Case 2
Signal plausibility with lean mixture (during fuel cut-off)
Enable
Conditions
Coolant temperature >73?(163?)
Downstream O2 sensor operative readiness detected
Downstream O2 sensor pre-heating phase finished
4g < Integrated Mass Air Flow in last Fuel Cut-Off
Downstream O2 sensor signal at end of last fuel cut off <0.3V
Integrated air mass flow in Part Load >150g
Minimum upstream O2 sensor voltage during Catalyst Purge phase >0.6V
Modelled catalyst temperature >400?(752?)
No relevant failure
11V< Battery voltage <16V
12g < Integrated mass air flow since fuel cut-off begin <250g
10V< Battery voltage <16V
Threshold
Value
Max. downstream HO2S <0.6V during catalyst purge after fuel cut-off
Downstram HO2S >0.3V during fuel cut-off phase
Diagnostic
Time
3 valid fuel cut-off followed by catalyst enrichment phases
5 sec.
MIL On Condition
2 Driving Cycles
? Sector A : Signal Normal
? Sector B : Signal Fluctuation
? Sector C : Signal Recovery