Mazda 'Check Engline Light' Issues
Repair Advice for 2000 Mazda 626 with a Trouble Codes P0421 and P043
Your auto repair questions answered by car repair expert Vince Ciulla about a 2000 Mazda 626 with trouble codes.
This weekly Q&A session answers your questions about cars and trucks. This week explores twin Mazda 626s with the same problems.
Question:
Vince, I own two 2000 Mazda 626s that my wife and I bought them on the same day. Both cars get the "Check Engine" lights under the same conditions and I obtained an OBD II code reader to see what codes were set. When traveling on the open road at high speeds, the light never goes on. It only lights up when traveling locally at around 35 MPH. One car has over 170,000 miles and got codes P0421 and P0431 (Warm Up Catalyst Efficiency Below Threshold - Bank 1 / 2). The other car has over 190,000 miles and only got code P0431.
Note: I had all the oxygen sensors replaced on the 170,000 mile car prior to getting the OBD II reader with no change in getting the Check Engine light.
Is this totally a problem where the catalyst converters have to be replaced or do you think there might be another factor that could cause this condition?
Thank you,
Richard
Barnegat, NJ
Answer:
Well Richard, the problem is not with the oxygen sensors -- quite the contrary, as they did exactly what they were supposed to do. The reason they use an oxygen sensor before and an oxygen sensor after the catalytic converter is to check on the catalytic converter. By comparing the exhaust gas before it goes in and when it leaves the Powertrain Control Module (PCM) can determine if the catalytic converter is working properly. In this case the PCM has determined the catalytic converters are not working properly and turned on the Check Engine Light (MIL).
Catalyst Efficiency Monitor Description:
This monitor checks for deterioration of the catalytic converter by comparing upstream (front) and downstream (rear) oxygen sensor (HO2S) readings. When the signal from the downstream oxygen sensor is similar to the signal from the upstream oxygen sensor, the PCM judges that the catalytic converter has deteriorated which illuminates the MIL and stores a DTC.
Under normal closed loop engine operation with a good catalytic converter, the oxygen storage capacity of the catalyst is high, so the voltage of the rear HO2S will switch slowly. With a deteriorated catalytic converter, the rear HO2S will fluctuate rapidly near the same frequency of the front HO2S. Refer to the illustration showing an efficient catalyst and a deteriorated catalyst.
The efficient catalyst on the left has a switch rate or several inversions across the 0.45 volt threshold on the front HO2S while the rear HO2S has few inversions across 0.45 volts. The inefficient catalyst on the right has the same number of inversions on both the front and rear HO2S.
- Counting Switches: Front and rear HO2S switches (switches past 0.45 volts from rich to lean) are counted. After the required number of front switches is counted, the front to rear switch ratio is calculated. A new or efficient catalytic converter will have a switch ratio near 0, while a failed catalytic converter will have a switch ratio of 0.8 to 0.9.
- OBDII Drive Cycles: A moving average is used to judge catalytic converter condition, so it may take up to six OBD-II drive cycles for the MIL to illuminate for a deteriorated catalytic converter.
- Conditions: This monitor only runs after the following conditions are met: the engine is at normal operating temperature based on good inputs from the ECT or CHT, ambient air temperature is not extreme (based on IAT), the engine is operating at greater than minimum load (based on MAF), vehicle speed is within a specific window (based on Vehicle Speed Sensor VSS), and the vehicle is driven at part throttle acceleration (based on Throttle Position Sensor TPS).
So, what you are looking at is replacing the catalytic converters on both vehicles. You will probably find that both vehicles are California models. Non-California codes for the same thing are P042 and P0430. P0421 and P0431 are for California. California models wind up in NewJersey because New Jersey has stricter emission laws than California does.
That's all for this week. Feel free to browse the archives for past topicsand other resources. And, as always, if you are stuck and need an immediate answer you can always call Vince on the phone.
