Audi 2002 Allroad AKE - MAF Sensor Failure

[ceilidhalfie]

Just thought I would share a worrying week with you. Last Sunday my wife returned from a trip to Blackpool back to Scottish Borders and car ran absolutely fine. 30 minutes later I got into the car and took off - IN LIMP MODE. Travelled about 6 miles and crawled back home.
Monday morning, ran the full VCDS diagnostics - no fault found. Suspecting vacuum issue, I replaced the N75 valve with a spare I had but did not resolve. Then took it to my local (very good guys!) garage who kindly gave me a courtesy car until they could have a look at it on the Friday. Their diagnostics showed no faults either. After checking pipes and hoses AND removing/checking the turbo, still no joy until they decided to disconnect the electrical connector on the MAF sensor. BINGO - car roars back into life.
The garage managed to get a used MAF unit for £39 and the car is not only running great again - but also passed its MOT too.
So beware, certainly on my car a MAF sensor failure does not show up on the diagnostics.

[Crasher]

It used to be a well known symptom, no codes, no power (therefore it is not in limp mode) then fit a new Air Mass Meter. As this problem died off years ago, people seem to have forgotten about it, these days VAG AMM’s are very reliable.

[Amphitryon]

I think this is a useful reminder of the limits of self-diagnostics.

When looking at the output of a single sensor the only way the relevant control unit can be sure the sensor is faulty is if the output of that sensor is impossible in normal operation. So for example, for a sensor that works on some kind of resistance if the normal operating range is 100 to 200 ohms and the value being read is close to zero then clearly the sensor is playing up (short circuit). Likewise if the resistance is very high then again the sensor is faulty (probably open circuit). But if the normal working region was between 0 and 100 ohms and there is a short circuit that means it always reads zero then how is the control unit to know this value is false?

That sounds like what was happening with the MAF. It was giving a plausible but incorrect reading indicating that very little air is entering the engine so consequently a very small amount of fuel was injected to avoid burning rich and generating clouds of soot and thus there was very little power.

Of course, when more than one sensor is taken into account there are more opportunities to spot malfunction because the control unit can determine that the output of sensor X is incompatible with the output of sensor Y so one or both of them must be faulty but even so there will always be faults that the control unit is unable to detect.

[Crasher]

The latest models have an "Air Mass Meter plausibility test" function in basic settings but I have never had it show me one is faulty as they don't appear to be failure prone anymore.