Weird Check Engine Light..

[Hillbilly]

This has been going on for quite a few years now, but thought I'd try to find a direction to look. The check engine light comes on when the ambient temperature gets above 45 or so, but in the winter when it drops down around freezing or so, it goes off and stays off until it warms up to 45-50 again. Truck starts and runs the same, gas mileage is the same, nothing noticeable changes wit the light on or off. Being a '94, it's pre OBD2, and the only thing I could gather from the analog volt meter check was maybe an O2 sensor. But not real sure on that and don't have a working analog meter anymore.
Have never heard of an O2 sensor that worked or didn't work due to outside temperature. Anybody experience this problem before? And if so, what was it? :scratchheadyellow:

[royster]

I had a similar situation with my '90. Replacing the throttle position sensor kept the light off, but I also replaced the temperatur sending unit...the one for the ECU, not the gauge. It sounds more like a faulty sending unit for temperature...a lot less costly: the TPS runs $135.00, (Rock Auto) though they eventually need replacing.

I also replaced the O2 sensor...all of this when I did a minor rebuild.

Not sure if the codes are the same for 94 and 90, but the code also indicates fuel pressure (check the fuel filter). An analog meter is $10 at Mal*Wart, you might as well invest in one so your truck can "speak" to you.

[Hillbilly]

I had a similar situation with my '90. Replacing the throttle position sensor kept the light off, but I also replaced the temperatur sending unit...the one for the ECU, not the gauge. It sounds more like a faulty sending unit for temperature...a lot less costly: the TPS runs $135.00, (Rock Auto) though they eventually need replacing.

I also replaced the O2 sensor...all of this when I did a minor rebuild.

Not sure if the codes are the same for 94 and 90, but the code also indicates fuel pressure (check the fuel filter). An analog meter is $10 at Mal*Wart, you might as well invest in one so your truck can "speak" to you.

I'll see if the local Wally World has an analog meter. I have a fine digital one, but no good for this. Next time I make a Rock Auto order I'll pick up a temperature sender too. Is the one in the thermostat housing the one that feed the ECU, or does it do the gauge?
Thank you for the feedback, much appreciated!

[royster]

The volt meter will be over in tools...I don't think they have them in automotive.

I'll need to check my service manual for which sending unit is which.