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vwbugman, VW Technician
Category: VW
Satisfied Customers: 821
Experience:  35 years experience on VW air cooled and water cooled repair and restoration
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2000 VW: Beetle..Air Filter, Check Engine Light came

Customer Question

2000 VW New Beetle, Check Engine Light came on (Trouble Code P1128), Replaced Mass Air Flow Sensor and Air Filter, Check Engine Light came on again after driving 30 miles (put in gas once) (Trouble Code P0172)
Submitted: 8 years ago.
Category: VW
Expert:  vwbugman replied 8 years ago.
Did you use an oem maf sensor? How many miles? 1.8 or 2.0?
Customer: replied 8 years ago.
Mileage: 55922
Engine: 2.0L gasoline
MAF:     Should be an OEM (Bosch). The job was not done by a dealer, but an independent repair shop.
Expert:  vwbugman replied 8 years ago.

There are several things that can throw the code.


Check for vacuum leaks

Loose gas cap

Catalytic convertor stopping up

Faulty mass air flow sensor

Leaking injectors

Fuel pressure regulator

Water temp sensor


You need to have the mass air flow checked with a scan tool such as VAG-COM to check the readings . Aftermarket sensors seem to be a problem.

Customer: replied 8 years ago.
How likely is it that the cause is the loose gas cap?
(I have tightened the gas cap and driven it twice, around 5 miles after the check engine light came back on. Because it seems that if it is the loose gas cap, it is self-correcting.)
I am planning to call the mechanic and probably take the car back to the shop and I don't want to make a fool of myself. What should I ask the mechanic who replaced the MAF? And what am I entitled to?
(If he said something like "Oh, it's the leaking injector this time", should I accept it and pay the repair charges?)
Customer: replied 8 years ago.
Or should I just cut the loss and bring the car to the dealer or another shop?
(Then I have to explain to them the first MAF replacement.)
Expert:  vwbugman replied 8 years ago.
I think i would take it to a qualified vw repair shop or dealer and invest the hour of labor to diagnose. It is too expensive to just be guessing. If the shop you deal with now does not have vag-com or a dealer scanner, I doubt they can get a proper diagnosis. The problem is that this code is very hard to diagnose. You don't have to accept the dealer repair but it would cost you the diagnosis time. Also request that the dealer stand behind the repair if you do decide to do it. It is probably something that will not cause you any issues if you decide not to repair. A lot of times the vw codes will come on with just a hiccup. VW's are extremely sensative. Also if there is a catalyst problem, there were some extended warranties on the cat system on the aeg engine for your year model if you have the aeg engine.
Customer: replied 8 years ago.
Could you explain more about "request that the dealer stand behind the repair"?
The mechanic I am dealing with now also gave me a one-year warranty.
But you can always say that the repair done is totally fine but there is another reason when the check engine light comes back.
Customer: replied 8 years ago.
And please give me some advice on:
1. How likely is it that the cause is the loose gas cap?
2. If I go back to the mechanic, what am I entitled to?
Expert:  vwbugman replied 8 years ago.
gas cap not likely
second question: If the maf is bad he should replace it under warranty. If it is bosch oem he should have a 1 year warranty. The problem is he may not have the equipement to properly check it. A scan tool such as vag-com has the capabilities of checking the air flow sensor measurements to see if it is within specs. It could be the catalyst or something else it could be are oxygen sensor malfunctions. It's too expensive to just guess. I would go to a dealer and invest the intitial amount to diagnose it. You don't have to have them repair it if it is too expensive. You can then look for an independent to do the job. This type of code is very hard to give a certain diagnosis. Especially over the internet without checking it with the proper tools.
Don't worry about accepting if you are not satisfied with the answers provided or if you are uncomfortable, I can opt out and let someone else give it a try. Just let me know.
Expert:  vwbugman replied 8 years ago.
Also make sure there are no vacuum hoses cracked or broken. You have to inspect them closely.

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