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Old November 20th, 2011, 17:27   #1
miketoth1979
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Default P0402, EGR Excessive Flow, and my solution

Last week, my engine check light on my '01 5sp Golf illuminated while I was driving, and when I scanned the car, the error code read "P0402 - EGR Excessive Flow" The performance of the car was strangely not affected at all. No limp mode. Plenty of power, as always.

This struck me as odd, so I did a bunch of research on this site. Turns out, the EGR doesn't directly measure its own airflow, but instead uses an indirect measurement based on what the Mass Airflow Sensor (MAF) is reading, or some such nonsense. To get the P0402 code to show up, one of the following things was happening:

- The EGR was not opening/closing, maybe a leaky diaphragm in the EGR itself
- The EGR was not opening/closing because the N18 valve wasn't working
- The EGR was not opening/closing because of some other vacuum leak

OR

- The EGR really WAS opening and closing, but the stupid MAF sensor was measuring incorrectly, which was confusing the ECU when it didn't get the numbers it expected, which caused it to throw the engine code.


I checked for vacuum leaks but found none. N18 was working just fine. Since I have a bit of a weepy EGR, I wondered if this had anything to do with the problem...

Turns out, it doesn't, and it was just a dirty MAF sensor. I used some MAF sensor cleaner and gave it a thorough spraying (but NO scrubbing). Popped it back in the car, started it up, and no more engine code for the past several days and many miles. Glad I found that out before I spent any money on new valves, hoses, MAF sensors, etc!!

Moral of the story: code "P0402- EGR Excessive Flow" is bogus. If the EGR system really did have a vacuum leak, wouldn't the EGR be stuck closed? And that wouldn't be excessive flow at all...it would be no flow. So if you get this engine code, check your MAF, because it could be the culprit.
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Old January 26th, 2012, 10:07   #2
copbait
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actually it is excessive flow at the maf not the egr, which is what happens when your egr is stuck closed, it is not pulling air for the cleaner because it is coming from the exhaust. But congrats on the cheap fix, I am never so lucky
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Old January 26th, 2012, 10:12   #3
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Yes, since the ECM is measuring EGR using the MAF, when the EGR is not flowing you read an excess amount of air, hence "excess flow". Nice that you can fix an issue by just cleaning the contacts.
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