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So about a month or so ago. My Check Engine light went on. I checked the gas cap, but nothing changed so I took it in to have the codes read. Came back with 2 codes. One saying the fuel cap may be open (it wasn't) and one saying the thermostat stuck open, or thermostat sensor was going out... etc. So the guy checked my fuel tank cap, and it was getting kinda old, not clicking shut properly. He said, go get a new cap, ($25 fix), I'll reset it.... If it comes back on with the same code later, it is another issue. As far as the thermostat code, ( I can't remember the exact code #, its written down at home) he said, wait for it to come back on, quoted me like $250 to replace the sensor and the thermostat with labor. I thought that was far, ($100) for labor in San Francisco is great! But he said it wasn't URGENT, just to see when it came back on, and I could fix it then. I decided not to preventatively fix it and just wait till the light came back on. It came back on 2 days ago...
I've read all the posts about replacing the thermostat and cooling system on E46's, especially when they are at 105k miles like mine. So I made an appointment for tomorrow morning. Today on the way home from work, my girlfriend said the light never stayed on!! It is now off. This Confuses me because I didn't think that it would go off by itself? I thought that it had to be reset. Now I'm confused. Any thoughts? Thanks!
 

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the check engine light that is thrown by the thermostat(and sensor, they're both the same thing) just is telling you when the thermostat is sticking. It doesn't mean it's stuck and will make you overhead, it is just notifying you that your thermostat is now stuck open. You should also be noticing your car isn't warming up completely when you're on the freeway. I would get this fixed asap though, as the thermostat can both stick open AND closed.

btw, this is really easy to DIY. Thermostat is super simple.
 

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It'll go off after a certain number of drive cycles without the fault reoccurring. It's too late for me to go digging around in my manuals, so I can't say exactly how many. Two consecutive drive cycles with same emissions fault sets the light, after a certain number w/o fault the light will be turned off.
 

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The thermostat code doesn't throw a CEL (at least on my car). Loose gas cap does throw a CEL, which does reset itself after a while.

You still need a thermostat!
 

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I had a faulty thermostat sensor that turned on my SES light. It's not a pivotal fix as the engineers at BMW so graciously designed the valve to fail in the open position. Which means your car will run cool and you'll see a slight decrease in engine performance in the winter months due to the increased time required for your engine to reach its optimal operating temperature (if it in fact ever does).
 

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my car has started to out of no where misfire when im at idle at a red light (at least once a day it happens) i then proceed to turn the car of and then turn it back on and bam no more misfire till maybe the next day..a bit confused i erased my codes and will be waiting till she misfires again to read codes
 

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my car has started to out of no where misfire when im at idle at a red light
That is a hard one to call, could be sparks, coils etc. years back it would be something like a faulty coil, however given the age that these cars are creeping onto could be the cause of a number of things.


Given that you turn the car off and then on and the problem goes away make me think it is your O2 sensors. Easy fix, but they are really expensive for what you get. I remember similar and tries to do the one by one O2 sensor replacement, I ended up replacing them all.


TO THE OP:

The thermo will not throw any codes. The CEL (check engine) light will turn it self off after (i think) 3 cycles without throwing the the same code. In short the lights don't really ever go away quickly, and for good reason. You sure it was not the coolant level light she saw? that might come on and off quickly, that or the battery light. I doubt if it were the CEL that it would blink on and then off in a short ride.....
 

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TO THE OP:

The thermo will not throw any codes. The CEL (check engine) light will turn it self off after (i think) 3 cycles without throwing the the same code. In short the lights don't really ever go away quickly, and for good reason. You sure it was not the coolant level light she saw? that might come on and off quickly, that or the battery light. I doubt if it were the CEL that it would blink on and then off in a short ride.....
this is wrong, yes it will. There are specific thermostat codes that are thrown and will be shown by a check engine light. It will either say "pxxx thermostat stuck open" or "pxxx thermostat stuck closed"
 

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this is wrong, yes it will. There are specific thermostat codes that are thrown and will be shown by a check engine light. It will either say "pxxx thermostat stuck open" or "pxxx thermostat stuck closed"
This :thumbup:

I've got an intermittent Thermostat stuck open code, Peake Scan tool helped out there, and now I've got a new thermo coming in from bavauto :thumbsup:
 

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having the t-stat stuck closed is scary.... as scary as having the expansion tank burst and losing all coolant
 

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this is why you buy a peake tool from ken, i reset codes all the time that never come back, you dont read a code once and sell/buy a part.............well, not unless your in the business of selling parts ;)


always reset first time, no computer is without flaws
 
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