Fault Codes on '95 M3

[Dr. Car]

I have a constant CEL and the Peake reader gave the following fault codes for my '95 M3:

45 - Knock sensor, cyl. 1-3
46 - Knock sensor, cyl. 4-6
od - Oxygen Sensor

The car runs fine.

Recently I had a problem with the car not starting sometimes and that was solved (apparently) by changing from my aftermarket chip back to the OEM one. The car was run a number of times however with no CEL following that fix. However, I am concerned that going in and out of the DME may have damaged it, e.g., by static discharge.

Also, my modifications could have something to do with the problem. I have an ECIS cold air intake, Turner shorty headers and a custom 3" header-back exhaust with high flow cat. But all that stuff has given me 7000 trouble free street and track miles.

The car has 45,000 miles, with a lot of short trips in its history when used as a daily driver. Perhaps a lot of carbon buildup.

I'm not much of a mechanic and I'm eager to get this behind me and fixed right the first time. So, my instinct is to take it to a BMW mechanic and solve this potentially complicated problem with $$$. Really, I'm guessing the fault codes stem from the engine computer, and I'll need a new/rebuilt one which is at least $550 for the part. Then the mechanic is likely to say, hey 50k miles is time to replace your oxygen sensor and your knock sensors anyway. I hear the oxygen sensor is $150 full price ($80 on eBay) and easy to install I think, but knock sensors will end up being a bunch of labor and likely some "while you're at it" stuff like fuel filter, O-ring gaskets, hey let's put in new spark plugs, etc. $2000 wouldn't surprise me. (I saw a DIY explanation on this site for knock sensor replacement and that is WAY over my head.)

I'd appreciate any words of wisdom, such as what these codes likely mean, or how much cash this is likely to set me back.

[HansundFranz]

There's a lot of assumption here and very little information. The oxygen sensor can be checked with a scanner. The knock sensors can be tested by tapping on the engine block in their vicinity and looking for the ignition timing to retard. If the oxygen sensor proves faulty, then you can replace it and see if it has any affect on the other faults. There is no need to replace half the car along with the knock sensors unless you want to. Past trouble-free operation is no guarantee there will be no problems in the present and future. Otherwise cars would never break.