My girlfriend drives a VW Passat 1.8 turbo. This issue began simply enough. Her check engine light came on and the codes revealed that she needed a new pre cat o2 sensor. I picked up the part and replaced the sensor thinking all would be well, but several miles later, her CEL came back on and the car went into “limp” or “dummy mode”. I did another diagnostic scan and it is now saying that it running too lean and the pre cat o2 sensor (brand new), mass air flow sensor or large vacuum leak are to blame. I checked all visible vacuum hoses and could not find any leaks at all and I have a hard time believing that the MAF would suddenly fail several miles after replacing the o2 sensor. Am I missing something here? Has anyone had a similar situation that can provide some insight?
Does the air control on this work using vacuum? If so, if you turn on the air and aim it out the dash, does it come out there or out the defroster port (at the base of the windshield)? if it comes out where you set it, it’s not a vac leak.
It may not have been the pre cat sensor, but the line to the sensor, if it can’t get a reading, it doesn’t know.
Try this site for answers
Realblinky, that is a great idea to check the air control. I hadn’t thought of that. I’ll check this tonight. Also, I’ve now posted my question on passatworld.com and have my fingers crossed.
Also, it may be worth mentioning that after inspecting the vacuum hoses and not finding a leak, I reset her codes. The car again ran perfectly fine until the ECU cycled at about 60 miles and returned the car to limp mode.
Cars are fussy about oxygen sensors…Perhaps the new one is operating out of spec. Is it a genuine VW part or a “replacement” part?
This has worked for me… Spray the old sensor with “electrical contact cleaner” followed by compressed air. Repeat several times. Reinstall the old sensor and see how the car responds…
I’ll check into this as well. The replacement sensor was a Bosch that was pretty much identical to the one that I removed. I didn’t check the old sensor for a VW part #, but I would assume it was original. I plan to clean the MAF Friday evening and could clean up the old o2 sensor while the MAF is drying. Thanks.
I must say this is fairly strange to me. I’ve done the o2 sensors on my old Ranger many times and have never run into any issues at all after replacement.
Shepherd…
Somewhat unrelated, but important nonetheless…
Make sure your GF uses ONLY a fully synthetic engine oil, and ALWAYS does the oil/filter changes at no longer than 5000 miles. The 1.8T VW engine is VERY prone to sludging.
What year/mileage on this car?
For $30-$40 you can buy a USB to OBD-2 cable that comes with a simple diagnostic program for your laptop… Then you can monitor ALL the sensor outputs in real time as you drive…This takes all the guess-work out of it.
Repair shops, capitalizing on motorists ignorance, charge hundreds of Dollars to perform “A complete diagnostic scan” when, for $40, you can do it yourself, whenever you want to…
Caddyman…
Do you have a speciffic recom. for that? I think I might be interested in one. Thanks.
That would be a deal IF it could read VW codes - most likely it won’t .
You need a VAGCOM to read VW codes .
OBD-2 systems are pretty much standard (by law) throughout the industry…
She is currently running synthetic and is surprisingly diligent about changing it on schedule. She doesn’t listen to much I say, but somehow that has managed to stick in her head haha. Her car is an '03 with roughly 78k on it. Ran like a watch until a couple of days ago. I’ll have to get to the bottom of this one ASAP as she has an emissions test coming up.
There are 600 similar items on e-bay. Just search “OBD2 USB” to find MANY others…