I had such an interesting experience with my recent CEL on the 2000 a4 ATW (120,000 miles) that I want to document it for some future member with similar problems. Picked up a CEL and read codes P1128 and P 1113. Most members suggestions were "vac leak" or " bad MAF."

My old vac lines were dry rotting so I replaced 8 feet of small (5/16 or was it 5/32??) line. Codes cleared and CEL came back.

I then made the PVC pressure tester and used it. I rigged my garden sprayer to pressurize the system (not much control with it...) and "found" a large leak at the half moon gasket near the firewall end of the motor which is the cam chain tensioner gasket. Did I find this leak or blow the gasket out with my fancy pressure testing equipment? I ordered the gasket (and valve cover gasket) but in the mean time I used a putty knife to fill that half moon cavity with blue RTV (the gasket was still in there). Worked like a charm. But, codes cleared and CEL came back.

More pressure testing. I could hear a VERY faint hissing sound coming from the IC. My hope was that it was too small to trigger the CEL. Could it be the MAP fitting?

I broke into the pre cat Oxygen sensor plug on the fire wall and read "some" resistance across it (sensor side). Since it showed some resistance I assumed it was ok and not "burned out" and thus open.

Screw it - let's dump another $100 and get a MAF. From Advance ordered on line. Installed with ease, and now have 500 miles without CEL.

Runnng great (for stock) and the data center says 27 mpg average, which is a combo of highway/city.

 

good stuff....

hearing stuff like this gives me confidence to do my own maintenance and not bother with mechanics.