CACKYARD MECHANIC: Problems with mass airflow sensor code

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This article was published 21/05/2010 (5673 days ago), so information in it may no longer be current.

QUESTION: I have a 2001 A6 with 4.2l V8. The ‘Check Engine’ light has been coming on intermittently. Lately it remains on all the time. The local Audi dealer reset the codes and I changed the mass airflow sensor. That did not fix it though, so later on I looked into it myself with a friend who has OBD software for my car. With the help of Audi forums, we found it was again the mass airflow sensor’s fault and it’s an intermittent issue that even forum members did not have a sure fix for.

The car is driving OK, though, and I believe it may be due to my catalytic converters as they are rattling pretty loud, especially when it was cold. I know the cats will need to be replaced soon, however it’s a rather expensive fix since there are two of them and even the Audi dealer recommended doing it elsewhere. My question is whether driving with crumbled cats is going to affect my car, and in what respect?

ANSWER: There are a couple things going on here. First, there seems to be a fault that relates to the mass airflow sensor. I assume you have found a fault code for the sensor during your diagnosis and that is what caused you to replace the sensor. Secondly, the catalytic converters appear to be damaged. Let’s look at the mass airflow problem first.

When a code is set for any sensor, that sensor may be faulty, but in most instances it is something else that is affecting the sensor performance and causing the code to set. Replacing the sensor won’t fix the problem if that is the situation. A mass airflow sensor code can be set any time there is an air leak between the sensor and the engine intake. The computer compares engine vacuum and rpm to mass airflow readings. If the two values don’t match, a code is set. A broken vacuum line, incorrectly operating crankcase ventilation valve, loose ducting, or even an evaporative emissions purge solenoid staying open could cause a mass airflow code to be set. When the code sets, the check engine light comes on. Look for other problems rather than blaming the mass airflow sensor. It could even be a poor electrical connection at the sensor or computer or a faulty manifold vacuum sensor.

It may sound hypocritical for me to give advice on forums, but my experience with many car-related mechanical forums is there is a huge amount of incorrect repair information out there. Most people have good intentions, but have little mechanical experience or training, so their advice may be worthless or even dangerous. There is some excellent repair information out there, but it can be difficult to identify. If you find a forum submitter that gives detailed information, examples of how to perform a task and provide clear answers to questions, there is a good chance it is correct, but still use that information with discretion. I include myself, as even good mechanics make mistakes. The good ones just keep working at it until they find the solution.

Finally, driving with damaged catalytic converters will create excess tailpipe emissions but as long as the exhaust system isn’t plugged it won’t affect engine operation. Depending on the year of vehicle, you can get the check engine light on all the time because the converter has failed the computer diagnostics, but even with the light on and a code set for the converters, the engine will still operate fine.

QUESTION: I have 2000 E350 with 7.3 L engine. Periodically, when I start the 7.3 the parking brake light stays on. This does not happen all the time and with no correlation to weather conditions. When the parking brake is on, cruise control will not engage. I appreciate any help.

ANSWER: Three things turn on the "parking brake" light. There is a switch on the park brake, a low brake fluid level switch in the master cylinder reservoir and, on your diesel engine model, a low vacuum switch on the brake booster. Disconnect one of these switch connectors at a time to locate which one is turning on the light. Because the brake light circuits are connected to the cruise disable circuit, the cruise problem should stop when you correct the parking brake light problem.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net