Just few days before I wanted to take Audi A4 B5 1.6 (year: 2000) to 'preventive' service. I've started the car normally, only the motor sound was very weird. So I tried to make a test drive. I couldn't reach any higher/normal speed. From the car exhaust flowed probably pure petrol(wasn't mixed?!) which everybody could smell.

A friend adviced that I should replace the oxygen sensor (lambda sonda in our language). So I order a new one and replaced it. But that still didn't work. So I scanned it with the VAG com tool, which returned the results on the image attached.

Which of those (if, of course) are related directly to the issue, I've described?

p.s. sorry for my bad english and (possible) bad technical knowledge.

enter image description here

1 Answer 1


First of all, no worries on the bad English side of things ... even those of us who speak it have issues sometimes! '-)

As to your issue(s) ... the first thing I do when I've encountered a large number of codes as you've gotten is clear the computer, then rerun the engine to see which codes persist. This gets you a lot closer to the root cause of the issue.

We can look at your codes and lump them into several categories. First of all, the P0420 code is saying your catalytic converter is working not so well. You said you smelled un-burned fuel, so this is how I'd expect your cat to be working. This is not your root cause and should be cleared up when we solve the issue (as long as it hasn't become saturated with fuel, which is another issue).

Next, looking P0300, P0302, & P0303 ... these all have to do with cylinder intermittent misfires. While this is an issue, the fact they are intermittent and on multiple cylinders (cyl 2 & 3), yet the vehicle is still running, tells me this is not the root cause of the problem, either. These, too, should clear up when you solve the issue.

Next, looking at P0172, which is telling us bank 1 is reading a too rich condition. It is reporting that the engine is running too rich, which is what we'd expect if smelling un-burned fuel out the exhaust. This, too, is not the root cause and should be cleared up when you solve the primary issue. You may find, however, since there was an extreme amount of fuel dumped into the system, the O2 (I'm assuming this is your new sensor as well) may have been damaged in the process. This may come back to life after the excess fuel quits dumping.

The P0134 code is saying your bank 1 sensor 1 has a null reading. Since your engine is a four cylinder engine, bank 1 is the only bank it has, and sensor 1 refers to the upstream (before catalytic converter) sensor. The write up from obd-codes.com states:

The powertrain control module (PCM) provides a baseline voltage of about 450 mV on the oxygen sensor signal circuit. When cold, the PCM detects the the internal resistance of the sensor is high. As the sensor warms up the resistance is lowered and it starts producing voltage based on the oxygen content in the exhaust. When the PCM determines that the time it took for the sensor to warm up is greater than one minute or that the voltage is inactive (not reading outside 391-491 mV it views the sensor as inactive or open and sets the P0134 code.

There might be an issue here, but at this point, I'd suggest when you clear the codes, this may or may not come back. If it does come back, it might also be the richness of the exhaust which may be causing the O2 sensor to not become warm enough in the one minute given. Still, it may be a good idea to check the connections to your O2 sensor and ensure the wiring is good. Also, check to see that the O2 heater circuit fuse is in good shape. Replace it if blown.

This leaves us with P0102, which has to do with the Mass Air Flow (MAF) sensor. I think this is where the problem lies, in one of two ways. Either the sensor is dirty or not connected properly, or it may be a combination of both. Since this happened all of a sudden, I'd lean more towards connection end of things than it being dirty.

My recommendation is two fold: First, check to ensure your MAF has the proper connection. The wiring may have been compromised somewhere between it and the computer. Double check all of it. Pull the plug and make sure it is clean inside (no corrosion) or bent pins. Since you are right there anyway, go ahead and clean your MAF with electronics cleaner. Be careful when you do so as the MAF is a delicate creature.

I think once you get the issue with your MAF squared away, your root issues will be resolved and your engine may start running right again. I say it may start running right again, that is as long as the overly rich condition hasn't harmed any sensors or the catalytic converter. They may clear up after the engine gets running good again.

If the MAF doesn't square your problems away, this may be a multi-problem issue. The next place I'd look is to see if one (or more) of your injectors may be in some state of stuck open. This would cause the very rich smell exhaust smell as well as the poor running.

  • Thank you for this great answer which made my look on the issue clearer. Those directions are very detailed and greatly described. I will check the steps.
    – balex
    Commented Jun 1, 2015 at 10:21
  • Please see that the fault in air mass sensor is also given as intermittent Commented Jun 1, 2015 at 11:37
  • @knocksAndMisfires - Thanks for bringing that up, as I didn't notice it. While the sheet says it is intermittent, the code is either fail or not. I don't think the intermittent has a great value in the diagnosis with regards to this code. Commented Jun 1, 2015 at 11:41
  • You must be right!!! Starting diagnosis with Air mass sensor Commented Jun 1, 2015 at 12:12
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    Just out of curiosity. What would happen if the air mass sensor is unplugged. Will the engine will go to emergency mode and the misfiring stops. If misfiring stops, then the fault P0420 will not occur? Commented Jun 1, 2015 at 14:39

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