Wheels: Ted M. writes by e-mail:

“I am hoping you can help me with a problem that has stumped me, a local VW dealer’s repair department, and my mechanic. I have a 2000 VW Passat (1.8L Turbo 4-cylinder with 5-speed manual transmission) that has the engine check light come on every time I start the car and after I reach 20 MPH or so, the ‘Emissions Workshop’ warning pops up. Both the dealer and my mechanic ran some diagnostics and the only error code that pops up is P1128. The dealer, my mechanic and I have tried various solutions to fix the problem — new gas cap, checking the air screen in the air box, checking the air filter cartridge, doing a smoke test for a vacuum leak, checking the mass air flow sensor, replacing two O2 sensors (near the engine and near the catalytic converter), and looking for an exhaust leak. After trying a fix and resetting the error codes, the check light stays off for a day or two then comes back on again. This has been going on for over a year. The car appears to run fine. The only real symptom we’ve noticed was some white residue around a replaced O2 sensor—my mechanic thinks that comes from the engine running lean but could not find a cause. Any suggestions would be appreciated.”

Halderman: Thanks for writing. This code means that the exhaust is too lean for too long. The main sensor that determines the air-fuel mixture is the mass airflow (MAF) sensor. As you likely know, this is a pattern failure item on your vehicle.

A couple of things come to mind:

1. Do you have an aftermarket air filter? If so, then this can cause contamination of the sensing element in the MAF.

2. When you checked the MAF was the engine tested for volumetric efficiency? This program can be found on at www.lindertech.com. Look at the heading labeled “downloads.”

If the efficiency measured at WOT and inputting the grams per second of air from the MAF sensor should be 100 percent or higher on your turbocharged engine. The MAF output should be higher than 100 grams per second at WOT. If not, then MAF sensor replacement is needed.

I hope this helps.

James D. Halderman is an ASE-certified master technician. Submit questions to jim@jameshalderman.com or follow him on Twitter @jameshalderman, or write to: Wheels, Marketing Publications Department, Dayton Daily News, 1611 S. Main St., Dayton OH 45409.