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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
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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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Old July 29th, 2014, 09:43   #4
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Default P0402, '02 TDI Jetta Wagon

Helpful post among all of those on the P0402. I'll resurrect this one.

I've got all of the same symptoms miketoth described - great performance, yet this code.

Car is 1 week new to me. I've ruled out N18 (12V got solenoid moving just fine), EGR is clean from previous owner, EGR diaphragm holds vacuum. There was an aftermarket boost gauge line installed that I removed and capped at the custom-made hole in the hard piping line between intercooler and intake. New vacuum lines installed where needed. The one thing I haven't done is focus on the MAF sensor.

When you say you cleaned the MAF, you mean you sprayed it's screen (electrical connection cleaner?) and/or the connection contacts? Could you elaborate? Thanks

Last edited by cumminsfromthecold; July 29th, 2014 at 10:23. Reason: clarity
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Old July 31st, 2014, 03:24   #5
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Quote:
Originally Posted by miketoth1979 View Post
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.
This is not true at all. I've encountered this code several times and everytime its been a bad diaphragm in the egr valve vacuum pod.
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Old July 31st, 2014, 06:30   #6
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Then why did cleaning the MAF fix the OP's problem?
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Old July 31st, 2014, 06:32   #7
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Quote:
Originally Posted by cumminsfromthecold View Post
...When you say you cleaned the MAF, you mean you sprayed it's screen (electrical connection cleaner?) and/or the connection contacts? Could you elaborate? Thanks
Electrical connection cleaner would be OK for the contacts, but there is a separate MAF cleaner product that would be sprayed on the MAF sensor itself. No need to disassemble the device, just make sure the spray is directed at the sensor.
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Old July 31st, 2014, 08:40   #8
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...and it's a hot film sensor rather than the more traditional hot wire sensor (needs a different can of cleaner... ie NOT the one with the picture of a a hot wire sensor on it )... and many will say spray-cleaning a hot film sensor is a crap shoot right from the get-go.
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Old July 31st, 2014, 11:17   #9
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Quote:
Originally Posted by Powder Hound View Post
Then why did cleaning the MAF fix the OP's problem?
Then why did it fix the ones I've had here? Mine we diagnosed with information gathering from the car. The valve doesn't open, the diaphragm was ruptured. The original poster never did any real info gathering. Did he pull the elbow pipe and see if the valve opened? Did he cycle the solenoid with vagcom to see if the solenoid applied vacuum to the EGR pod? Did he pump up the actuator and see if the diaphragm was ruptured. NO he just assumed it was a dirty maf and cleaned it. If he's right then fine, but I'll bet you have to clean the maf sensor at least a dozen times to fix a ruptured diaphragm.

Why did it fix his??? How should I know? (who cares)

Theory one. He posted that it was fixed then a few days later the code reoccured.
Theory two... His code was a fluke and clearing it would have been enough.,
Theory three... who cares.

But for every one reading this... certainly, clean your Maf sensor first, just dont be confused when the code keeps reoccurring. And if you have access to a mityvac then see if the diaphram is ruptured or maybe even make some of the checks I mentioned above.
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Last edited by CoolAirVw; August 1st, 2014 at 04:08.
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Old July 31st, 2014, 12:31   #10
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I'm signing up for more info and going to try some maf cleaner. You never know *shrugs*, it's cheap and worth a shot, at least.
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Old September 2nd, 2014, 04:27   #11
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So what happened?
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Old September 2nd, 2014, 07:07   #12
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Quote:
Originally Posted by Powder Hound View Post
So what happened?
With mine?
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Old September 2nd, 2014, 08:38   #13
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Quote:
Originally Posted by Powder Hound View Post
So what happened?
Send him a PM and ask him.

It would be nice if there was follow up.

Maybe,..,., been driving year and its worked the whole time. Car is fixed. No other work was done other than cleaning the maf.

or .... sorry I made this post as the code reoccurred 2 weeks after making the post. I eventually fixed it with a vacuum hose (or egr replacement).
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