Random Check Engine Light

A

Aebous

Elite
 Okinawa
Howdy,
I'm occasionally having my check engine light illuminate. This is during random times, can't reproduce it, over various roads & speeds(good, bad, motorways, villages). I've brought it to an indy a couple times, only to not have a code in the system. I've since been borrowing my buddies code reader (which claims to work on 2003 BMW's) to try to catch the code, but I haven't had the reader display a code. I'm about to order my own code reader (help in that area would be nice), but basically I'm wondering if anyone else has a had the check engine light randomly illuminate with no codes?

I don't figure it to be the gas cap, cause if that's loose the car has a seperate warning light to tell you.
 
just to clarify here. while driving the check engine light will illuminate... then extinguish while driving on the same trip?

Or does it turn on, and it disappears when you shut the car off?

does the car do anything funky when the light comes on? (i.e. misfire?)
 
ZED_NOT_ZEE said:
just to clarify here. while driving the check engine light will illuminate... then extinguish while driving on the same trip?
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Not on the same drive, it will clear itself after a bit of driving though. I've been borrowing a code reader from a buddy, however I looked online and his reader isn't listed as compatible with BMW's. So it hasn't picked up any codes but it can reset the light.

Or does it turn on, and it disappears when you shut the car off?
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Nope

does the car do anything funky when the light comes on? (i.e. misfire?)
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No, the very first time it ever happened the car did run rough, but every other time the light has came on the car runs smooth as butter.
 
sounds like a slight vacuum leak or a wiring issue. I had a problem similar on a previous 330i where the check engine light would come on and turn off intermittently. It was the wiring loom from the air flow sensor. Go over a bump and the sensor would not be registered by the computer, the check engine light would come on. After a while of driving the light would go back off.

Same thing too, scan for codes and nothing was in the computer. Had to bring it to a mechanic while the light was on and he scanned the code without turning the car off (while the light was still on).
 
Pete (Pete's Autotech Telford) scanned it with his reader, I didn't get the specific code but it was something to do with the thermostat electrical signal. Not too big a deal apparently, I asked how soon should I get it replaced he said he wouldn't wait until winter, but no rush to get it done right away.
 
If your coolant temp sensor is duff mate, its more of a reason for your bad fuel economy than the geometry :)

A bad one could be telling your ECU to constantly run under cold-start conditions.
 
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