I was driving my 540i today and noticed that the service engine soon light was on. The car is running fine and after the light came on i even got on the throttle a bit to see if it was acting funny or cutting out and its all good. When i got home i pulled the codes and got P0170 (fuel trim malfunction bank one) and P0173 (fuel trim malfunction bank 2). I'm just wondering what exactly it means and how i got about fixing it.

Thanks

Try clean the MAF sensor first.

Next if that doesn't work is an O2 sensor. The Maf is what could cause an O2 to read farther out than it can deal with, but if the Maf if good, the O2 sensor would be next for me.

If there's over 80k on the pre-cat O2 sensors they are a likely cause. If there's 100k on them then need to be replaced asap. There could be other problems (intake leak, bad CCV, contaminated MAF), but aged O2 sensors would be the place to start.

Here's a good thread on the subject (http://forums.bimmerforums.com/forum/showthread.php?t=448705).

I was thankful mine was just a loose MAF clamp.

If there's over 80k on the pre-cat O2 sensors they are a likely cause. If there's 100k on them then need to be replaced asap. There could be other problems (intake leak, bad CCV, contaminated MAF), but aged O2 sensors would be the place to start.


What is a CCV? It has 85k on it and to my knoladge none of the sensors were cleaned or replaced at all so possibally that is the problem?

Also, i filled up the tank yesterday and put a bottle of lucas fuel treatment in it, this wouldn't cause this would it?

CCV = CrankCase Vent, really just a fancy PCV. A crankcase pressure check will tell if it has failed. The crankcase vacuum should 3-4" of water at idle.

At 85k the pre-cat O2 sensors may have aged enough to cause some erroroneous enrichment of the mixture. But the odds are that they aren't the only culprit. An intake leak, bad CCV, or contaminated MAF could well be players also. And if the car has an oiled air filter (typically used for a CAI) the chances of the MAF being implicated go way up.

My 1999 540iT has approximately 112,000 miles and seems to have been well maintained. I bought it last Fall -- 3rd owner. I checked for previous service work, for which I found reasonably good records for 2003 - 2006, as well as CarFax reports indicating regular visits to a BMW dealer prior to that.

There are questions for the group at the bottom of this post -- what follows next is just the narrative of what happened.

I recently took the car to a good indie shop near here (Mercedes, BMW, and Volvo work). He found the P0170 and P0173 codes -- fuel trim on both banks. The car ran well, without any noticeable stumbles or hesitation.

He checked thoroughly for air leaks (found none), and suggested that he would try replacing the sensor in the MAF -- thinking this was the only thing common to both left and right banks.

That didn't clear the codes, though the car seemed to run well, without any hesitation or stumbling. We talked about the pre-cat O2 sensors being a common source of these codes, and (as far as I knew) they had never been replaced.

He replaced both pre-cat O2 sensors, and, voila -- the codes cleared and haven't returned. Incidentally, he swapped back to my original MAF sensor, and didn't charge me for that part -- just the diag time. He keeps a number of these in his local stock, just for this purpose.

There's one more code, which we're ignoring for now. I don't recall the value, but it indicates failure of the pump which periodically tests the evaporative emission control system for leaks -- by pressurizing the line.

Now questions for the group.

Why would both pre-cat O2 sensors throw the code at the same time? I realize the old ones were well past their normal life, but why would they fail at the same time? Or is it just that the CEL systems didn't detect the first failure when it happened?

And then the pump on the evap emissions systems also chose this very moment to fail (he tested the actual electrical connections to the pump -- it had power, but wouldn't operate).

Is this the Munich factory installed timers all causing components to crap out at the pre-determined time, gremlins, or just weird luck?