I just saw the check engine light come on on my 2004 VW Passat B5.5 and did a bit of research. Some people said that it would be a faulty oxygen sensor. I was about to order one, but then I read this (from here):

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.

That was in relation to another issue, however it confused me enough to doubt whether buying a new oxygen sensor would in fact fix the problem!

How do I fix the P0172 trouble code?

Also, if it is an oxygen sensor - are they all the same? Can I order the wrong one for my car or can I just get the cheapest?

UPDATE I got some more info off an app on my smartphone connected to my OBDII port. I see O2 sensor is N/A! That means that I can't see how lean the engine is running - that should mean it's the sensor that's broken now right?

Here's a screenshot:


Wow, sorry its so big!

  • P0172 can be caused by a lot of things. Use a scantool and make sure the sensor is working first. If it's an AFR sensor it may be too expensive to just swap out and try.
    – Ben
    Commented Oct 23, 2016 at 13:48
  • 1
    Also take a look at the freeze frame data. Is it rich only at idle? Is it rich only under load? Is the engine in closed loop? What exactly is happening when the code sets? Looking at the freeze frame first than performing a road test and capturing data is the best way to diagnose this problem.
    – Ben
    Commented Oct 23, 2016 at 13:55
  • Yes, a P0172 could indicate something other than a bad O2 sensor. The bad O2 readings are a symptom. Now you have to find the root cause of the problem. If you don't have a fancy scan tool, you can order one of those bluetooth OBD2 dongles ($30) and install the Torque app ($5). It will show you the numbers Ben is asking about. You could also order a new O2 sensor ($60?), pop it in, and see if the code comes back. I say "pop it in", but O2 sensors are not the easiest to replace. Commented Oct 23, 2016 at 17:34
  • you probably have an afr sensor and the app is pointing at the wrong pid. verify the type of sensor the car uses for fuel control and check your app to see if there are any other pids you can use. i'd expect if the o2s were broken that you'd be seeing reference voltage or a short to ground 0v and not a n/a. but i don't know how your app interprets data.
    – Ben
    Commented Oct 24, 2016 at 16:00

1 Answer 1


I see that this was asked a month ago and got no answers, but on the off chance the poster still needs help or somebody else comes across this, I'll do my best to answer.

If you have a bank that is running too rich, which is unfortunately a very generalized condition, one or more of the following conditions must be true:

  1. Too much fuel is being injected.
  2. Not enough air is coming in.
  3. There isn't enough spark to burn the fuel completely.
  4. The oxygen sensor is reading incorrectly, and everything is actually fine.

1 would generally have to be a leaking injector or a wiring/computer fault. The computer should automatically adjust the firing time of the injectors to achieve the proper ratio based on feedback from the O2 sensor. The fact that it's throwing that code means that it's trying and unable to adjust sufficiently to keep the air/fuel ratio correct.

2 is easy to check- make sure the air filter is clean and there are no obstructions in the intake path.

3 Check the spark plugs, check the plug wires, check the coils, check the wires feeding the coils. The Internet leads me to believe that your ignition might be coil on plug, in which case there are no plug wires. Generally a spark failure should be detected by the engine computer and logged as a misfire, especially if it were bad enough to cause a rich condition, so this one seems less likely. (I'm also assuming you don't have a diesel engine.)

4 could be the issue, but O2 sensors are a pain to access and require a special socket to remove/install, so I wouldn't just jump to that on a whim.

To answer your question about what to buy, very few things in the automotive world are universal parts. However, several manufacturers do use the same suppliers for some things like O2 sensors (Bosch is a likely candidate for those). Auto parts stores have huge databases containing information about all different vehicle models to cross reference what parts are compatible. You can either walk into a store and have them look it up in their computer, or you can search for the part on their web site, which will prompt you for as much information as it needs to narrow down your choices.

You didn't post which engine you have. The Passat has a lot of choices.

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