Check engine light - Emissions workshop

I have a 2000 VW Passat with 88,000 miles on it and recently the check engine light came on and I get a message on my dashboard computer of “Emissions Workshop”. I had the codes pulled at Auto Zone and this is what I got:

Long term fuel trim add. fuel B1 system too lean


Failed MAF

Probable cause

Low fuel pressure

Vacuum leak on engine

Failed HO2S (Heated oxygen sensor) Bank 1

I have not brought this into the dealer or a mechanic. The car runs perfectly, in fact, I seem to be getting a little better mileage.

Any ideas on what may be causing this and what needs to be fixed? I would like to be armed with good information before I bring it in for repairs.

Please post the codes, such as: P0303, etc.

At the top of the readout:

Troubleshooting P1136

That list of stuff pretty much contains the stuff that ought to be checked - although it seems to suggest that the code was caused by a bad mass airflow sensor (MAF). That isn’t a good assumption to start with but something to be tested.

I would:

  1. check for vacuum leaks
  2. clean the mass airflow sensor (MAF) and check its wiring
  3. test the fuel pressure
  4. have the o2 sensors checked with a scan tool
  • pretty much in that order.

Do you have an air filter other than stock? Cold air or K&N?

Not sure. Had my local mechanic replace the air filter after the CEL came on. I had read that a old air filter and stuff that had accumulated around the air filter could cause the CEL to come on.Don’t know what you mean by “Cold air of K&N?”.

A Cold Air Intake (CAI) system is simply a tube to the engine’s air intake with a typically conical filter on the end. It replaces the standard air intake. Cooler air is denser, so in theory one can get more oxygen molecules into the engine per volume of air (that’ll get more gas added too) and more power out. The true effect is more placebo than real based on everything I’ve read. K&N has emerged as the most common CAI system in the marketplace.

The reason Keith asked may be that K&N filters recommend a light oiling of the filter element, and overoiling has been known to cause contamination of the mass airflow sensor, which is downstream of the K&N filter.

It sounds from your reply like you probably have a stock air intake system.

Keith, did I get that right?

Is this something that an independent mechanic that services VW’s can deal with or do I have to take it to the dealer (which I am reluctant to do)?

Any independent should be able to deal with it - though one that specializes in VWs would certainly be a plus.

You can clean your own MAF sensor pretty easily.

To understand what’s WRONG, one needs to understand what’s RIGHT (or, normal). The air entering the engine gets measured by the MAF (Mass Air Flow) Sensor. Say, it measures 14.7 units of air. The air goes to the combustion cylinders where it gets burned with 1 unit of fuel. When the exhaust gases are measured by the oxygen sensor, its output (to the engine computer) indicates that the fuel/air mixture is normal (or lean, or rich).

What can go WRONG when things don’t follow the game plan:

  1. More air may go into the cylinders than the MAF reports to the engine computer. OR, the MAF may be correct; but, unmetered air gets into the intake after the MAF. Result: a lean burning fuel/air mixture.
  2. The fuel injectors may spray too little fuel (because they don’t spray long enough, or the fuel pressure is too low). Result: a lean condition.
  3. The reading by the oxygen sensor may indicate (wrongly) that the air/fuel mixture is lean. Result: the engine computer thinks that the fuel/air mixture IS too lean.

What to do? You check/test each part (MAF, fuel injection SYSTEM, oxygen sensor) to determine if one is not performing properly. You check for air (“vacuum”) leaks. You check fuel pressure under varying engine loads. A fuel injector tester can, also, tell how long a fuel injector sprays.
Lastly, you try to find a shop/mechanic who can/will do these things.