The catalyst’s efficiency is demonstrated by its ability to oxidize CO and hydrocarbon emissions. The Powertrain Control Module (PCM) compares the output signals of the front and rear oxygen sensors to determine whether the output of the rear sensor is beginning to match the output of the front oxygen sensor. Air/fuel mixture compensation keeps the frequency of the front oxygen sensor high due to the changes from rich-to-lean combusition. The catalyst causes the rear oxygen sensor to have a lower frequency. As the catalyst wears, the rear oxygen sensor’s signal trace begins to match the front oxygen sensor’s signal trace. That is because the catalyst becomes saturated with oxygen and cannot use the oxygen to convert hydrocarbon and CO into H2O and CO2 with the same efficiency as when it was new. A completely worn catalyst shows a 100% match between the frequency of the front and rear sensors.
If the correction value of air-fuel ratio is higher than the threshold value, PCM sets DTC P0171.
Item
Detecting Condition
Possible Cause
DTC Strategy
? Fuel trim limit
?Poor connection
?Related sensor
?Intake air system
?Fuel pressure
?PCM
General Enable Condition
? Coolant temperature > 70 degC
? Intake air temperature < 80 degC
? Throttle angle < 60 %
? Integrated air mass > 10 grams
? Closed loop control enabled
? No transient control phase
? No canister purge phase
Threshold Value
case1
? Additive long term value > 10 %
case2
? Multiplicative long term value 1 > 1.23
case3
? Multiplicative long term value 2 > 1.23
Enable Conditions
? Air mass < 24 kg/h
? Air mass 40-80 kg/h
? Air mass > 100 kg/h
Diagnostic Time
? Immediately