Running Cold/check engine light

I have a 99 zx2 that i have noticed the engine temp slowly coming down over the past 100 miles. One day when i was driving about 30 miles at 80 mph the check engine light came on. im almost positive it has to due with the temp just above cold. I am headed to Auto Zone soon to get codes check. Any one got some heads up info before i go. The vehical has about 75K miles.

Your thermostat is sticking open.

Ok i understand that much, but why do i get a check end light. How does the computer know that the thermostat is stuck open to give me a sevice eng light

Ok i understand that much, but why do i get a check end light. How does the computer know that the thermostat is stuck open to give me a sevice eng light

Computer reading variance out of range discrepancy between cyl head temp sensor, coolant temp sensor, incomplete combustion due to low cylinder temp causing high HC in O2 stream that can't be compensated, etc. The ECM is pretty smart. It knows when something just can't be right, even if it doesn't directly know what the problem is.

OK, I really appreciate the info. One more question though, Once i have changed the thermostat and problem fixed will the computer reset itself

Computer reading variance out of range discrepancy between cyl head temp sensor, coolant temp sensor, incomplete combustion due to low cylinder temp causing high HC in O2 stream that can't be compensated, etc. The ECM is pretty smart. It knows when something just can't be right, even if it doesn't directly know what the problem is.
Actually, its not quite as difficult as you make it sound, the zx2 is one of several vehicles equipped with a thermostat monitor(similar to misfire monitor, evap monitor, egr monitor,etc.)when the processor notices that your engine hasn't reached a set temp. in a set amount of time, it throws up a flag(actually a diagnostic trouble code>probably in your case a P0125)and if this fault occurs on two consecutive key cycles, your check engine light appears on your dash, and yes, your check engine light will eventually clear itself assuming that the concern is fixed and no other concerns have arisen since the light has been on.

Actually, its not quite as difficult as you make it sound, the zx2 is one of several vehicles equipped with a thermostat monitor(similar to misfire monitor, evap monitor, egr monitor,etc.)when the processor notices that your engine hasn't reached a set temp. in a set amount of time, it throws up a flag(actually a diagnostic trouble code>probably in your case a P0125)and if this fault occurs on two consecutive key cycles, your check engine light appears on your dash, and yes, your check engine light will eventually clear itself assuming that the concern is fixed and no other concerns have arisen since the light has been on.

Cool, appreciate the info. My experience is primarily GM, and I've been outta the industry for several years. Not up to speed on some of the newer sensing abilities and PCM routines. But I can usually limp through a problem. I need to renew my Chilton's mag subscription. Liked reading all that model-specific stuff. Thanks again.

P.S. Keepin' your PM on tap for future reference. Seems like you might come in handy.:iceslolan

Did replacing thermostat fix your problem? I had simuliar problem, autozone showed bad temp control. Your post got me thinking that a stuck thermostat also might show that paticulair code.

I changed the thermosatte and didnt fix problem. I changed the lower temp switch on the thermostate neck and it seemed to fix problem for about a day. Now the thing still runs cold and im at a loss. If any one knows how the temp sensor on the top of the thermostate housing plays a part or what it exactly does i sure could use the info.

I changed the thermosatte and didnt fix problem. I changed the lower temp switch on the thermostate neck and it seemed to fix problem for about a day. Now the thing still runs cold and im at a loss. If any one knows how the temp sensor on the top of the thermostate housing plays a part or what it exactly does i sure could use the info.

The sensor on top was the one i replaced...i disconnected the battery to reset the computer and everything seemed to be normal again. Check engine light never came back on and temp gauge reads just under half scale [i guess that's normal]. I was thinking the sensor you replaced was a coolant level sensor[the one hard to get at]...i could be wrong though.

Something to think about,

I don't know if a bad temp. sensor will keep the raditor fan from coming on or not.....but, if i had it to do over again i'd leave my defrost on just to make sure the raditor fan was running all the time. I blew a head gasket last weekend and the only thing i can think point a finger at- is the temp. sensor wasn't kicking the fan on when it was suppose to.


pat