So I've been throwing code 15 on my 92 3.1 Firebird, coolant temperature sensor, low temperature indicated. After doing a bit of research on this site I'm pretty sure it's my ground wire to the coolant temperature sensor that is bad. I put in a new CTS and thermostat, still get code 15. When I Bridge the sensor plug I get code 14 (CTS high temp indicated) and on the positive/yellow wire it reads the proper voltage (5v). So it must be the ground wire correct? I really don't want to take the throttle body and top if the engine apart to trace and replace the wires and plug. Could I just snip the black wire and ground the plug to something else, or would that just create more problems? Thanks

http://www.chevythunder.com/cts_diagnostic_pg2.htm

 

If you bridge the sensor plug and the ECM reads that situation correctly, that is if it sees a short which it then would interpret as high temp, then there's nothing wrong with the ground wire.

OTOH, if you had 5V at the yellow and you shorted the plug and STILL got a low temp error, then the ground side WOULD be the problem.

Measure the voltage at the black wire at the plug, with it normally connected and at a normal kind of temp. Should be arbitrarily close to 0V obviously. If it is, then the ground wire is OK.

The voltage at the high side should be somewhere between 5V and 0V, depending on the temp. It should be near 5V when cold and nearer 0V when hot; probably 1 - 2V or so at normal running temp. If it's not something rather like that, then the sensor may be bad.

I'd recommend NOT messing with the wires in any case. Only as a VERY LAST RESORT, after conclusively troubleshooting (i.e. measurement of ohms from black pin of the plug to ground is something other than 0 ohms) ONLY. ABSOLUTELY NOT based on what you've posted so far.

Might just be something as simple as, the contacts in the plug are corroded, and it needs a new pigtail. If you change that out, solder the wires well and neatly, and use heat shrink & tape over the joints.

 

Everything Sofa said is spot on , it very well could be corrosion on the connector preventing good contact . Now , if per chance it isn't corrosion , something else to consider is this ;

NEVER , EVER , "assume" that "Brand New" equates to "perfectly functional" ! The only proof of a perfectly functional part is to see it actually preforming in it's designed for application .

You have no idea in my mechanical career how many brand new out of the box parts I've seen that were just flat out defective . A lot , and enough that I encourage folks to never make that assumption ; "I took it out of a brand new box , it HAS to be good !"

Even if your new CTS is good this time , In the future It's always something to keep in mind when all troubleshooting keeps pointing back to that brand new part that just maybe this is the 1 in 500 or so brand new parts that earn the "factory defective" name .

I wish you well with your further troubleshooting .

 

Thank you I interpreted the problem solving flow chart incorrectly, I see what you're saying.

It won't stay running long enough to get up to normal operating temperature, after about 5 to 10 minutes of running fine the service light comes on then it runs rough and stalls out (with pigtail plugged in the sensor/code 15 appears) I'm not sure how to get readings off the circuit while it's plugged in. But I suppose I don't need to, it's got to be the connector or a bad new sensor from what I understand.

Ok so I'll cut and solder in a new pigtail if that doesn't work I'll try another sensor, thanks guys.

 

You can measure the resistance of the sensor with your ohmmeter.

Should be several k ohms when cold like maybe 3k or more, a few hundred to maybe 1.something k when warm/hot. If memory serves it should be around 700 ohms at 200°F, more or less.

Measure that first. If it's not in that ballpark, then it's bad. If it is, then it needs the pigtail. Not the other way around. Check the sensor first.

OBTW: the voltages I gave you earlier are what they should be with it plugged in and normally operating. Measuring the voltage with it unplugged isn't particularly informative.