Tech Tip: Diagnosing a VW/Audi MAF Failure – UnderhoodService

Tech Tip: Diagnosing a VW/Audi MAF Failure

One of the most common failures on 2000 and newer Volkswagen and Audi cars is the mass air flow (MAF) sensor, but properly diagnosing such a failure can be problematic.

One of the most common failures on 2000 and newer Volkswagen and Audi cars is the mass air flow (MAF) sensor, but properly diagnosing such a failure can be problematic. Your customer comes in telling you that his car just doesn’t have the zip it used to have. The car may or may not have fuel trim-related fault codes, most often 17536/P1128. You can’t readily determine that the car is down on power because you have no point of reference, as you haven’t been driving the car. So how do you help your customer?

In all likelihood, your customer is not dreaming. The MAF may be dying a slow death and under-
reporting air flow, but the values it’s reporting are not off enough to trigger a fault code pointing directly at the MAF. VAG repair manuals show specifications for MAF values in grams per second at idle, but checking at idle will rarely reveal a problem.figure 1

So, how does one determine that the MAP has failed? The first thing to do is look at the additive and ­multiplicative fuel trim values. These are almost ­always found in Measuring Block Group 032. An actual example is shown in Fig. 1. When a MAF is failing, the additive fuel trim will normally be within spec, but the multiplicative trim will be a positive value outside the specifications. Conversely, if additive fuel trim is out of spec, chances are the car has an intake (vacuum) leak rather than a failing MAF. Note that clearing fault codes (even if no fault codes exist) will also reset the fuel trim values to 0, so it is essential to do this check before clearing any fault codes. Figure 2

Of course, other factors can also throw off the fuel trims, so before replacing the MAF, it’s a good idea to do a full load test on it. This can easily be accomplished with a tool such as VCDS, which can log, graph and even show peak values during a road test. Check the output of the MAF sensor while doing a full-throttle run in second gear, as shown in Fig. 2. On most current VW/Audi engines, the actual MAF value can be found in Group 002, field 4 or Group 003, field 2. Assuming this test is done at or near sea level, the peak output of the sensor in g/s should be roughly 80% of the rated horsepower of the engine.

For example, if the engine is supposed to produce 200 hp, a healthy MAF is expected to produce a peak value of about 160 g/s. If the number is significantly less than that, you can be almost certain that the MAF is failing.

Exception: If the test is conducted in a location where the altitude is substantially above seal level on a non-turbo engine, an altitude correction factor will need to be applied. If the engine is turbo-charged, you should verify that it is producing the expected amount of manifold pressure (MAP). Requested and actual MAP can usually be found in Group 115 of the measuring blocks.

Courtesy of Ross-Tech, LLC.

You May Also Like

Alternator Testing For No Charge Conditions

Many alternator problems turn out to be nothing more than a bad connection at the alternator or a bad wiring harness.

If only battery voltage is present at the battery on a running engine, does this mean the alternator is “bad?” No, it does not. It only means that the alternator is not charging, but does not reveal why. Therefore, it does not point to a faulty alternator. All too often, the alternator is condemned by technicians due to this test alone. The cause could instead be a module that distributes the power to the vehicle and regulates the alternator.

Tech Tip: No Start in Cold Weather Due to No Weep Hole

This condition can be caused by condensation freezing in the muffler assembly.

Determining Catalytic Converter Efficiency

Almost every part on the engine determines how long a catalytic converter will last.

Talking To Customers About Oxygen Sensors

One of the toughest components to sell as a maintenance item, or even a replacement item, is an oxygen sensor.

PCV System DTC

PCV systems are far more complex now, but they are also much better at controlling crankcase pressure.

Other Posts

Hybrid Vehicle Fluid Maintenance

There are opportunities your shop can’t pass up just because the car or SUV has a hybrid badge on the back.

Using a Scope

Ignition coil and plug diagnostics.

Scope-diagnostics
Repairing Wiring Harnesses

A guide to probing and poking.

Diagnostic Procedures For Stop/Start Systems

Stop/start technology is more than just a switch wired to the brake pedal and starter.