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 | ● Long-term fuel trim limit (an excessive fuel supply) | ● Poor connection ● Related sensor ● Intake air system ● Fuel pressure | |
General Enable Conditions | ● Coolant temperature > 70℃ ● Intake air temperature < 80℃ ● Throttle angle < 60% ● Closed loop control enabled ● No misfire ● No canister purge phase ● No error on the related sensors (CKPS,TPS,ECTS,IATS etc) ● No error on canister purge valve, B1S1 | ||
Case1 | Enable Conditions | ● Engine speed : 1200~3800 rpm ● Engine load : 30~99.8 % ● Air mass : 40~190 kg/h | |
Threshold Value | ● Correction value during driving > 1.23 | ||
Diagnostic Time | ● 30 sec | ||
Case 2 | Enable Conditions | ● Engine speed ≤ 920 rpm ● Air mass ≤ 24 kg/h | |
Threshold Value | ● Correction value during idling > 7.5 % | ||
Diagnostic Time | ● 20 sec | ||
MIL ON Condition | ● 2 driving cycle | ||
