i was driving and i turned the a/c on and it blew hot air the other day, then tonight i went to check my coolant temp and all it said was ---F* whats going on the a/c and coolant temp display are shot, can anybody shoot me a couple ideas for a fix?

if not ill just have it warrantied lol

 

Sounds like the coolant temp sensor is bad.

 

^x2.

 

thats what i thought, i guess its warranty work, sounds llike i wont have a car for a week or so my stealership is uber slow with warranty work

 

part costs like 15 bucks probably takes 15 minutes to do if you would rather pay for the part and do it yourself.
What engine do you have?

 

Yeah, this should be an easy part to get plus maybe 15 or 30 minutes work. If it takes the dealership more than an hour or two total time then they really suck.

 

^^^^Agreed

 

Actually its not the ECT sensor.. Its the thermostat being stuck open. It just hasn't had a chance to set a code and turn on the SES light yet.

I've had this exact same thing happen on my previous Cobalts. The thermostat fixed it. It should be setting a P0128 for Coolant Below Thermostat Regulating Temperature.

 

Actually I concur it is the ECT. Malaclypse you missed a symptom, the A/C stopped working. That only happens if the ECT is read below freezing. Change ECT Sensor. Dealer may backprobe it before changing just to verify it's not a wiring issue elsewhere and waste time changing a sensor without need. Expect it to take an hour.

 

well magically i turned my ac on forgetting it was broke cuz it was so hot out today and it worked?, so i checked my coolant temp display and sure as **** it was reading hmm my cars a wierdo i guess i got lucky

 

It will die one day eventually. Hopefully it will do it again before warranty is up, if not it is an easy fix.

 

I didn't miss that symptom at all.

You can believe what you want but what I posted is exactly what's happened to my past two cars in addition to several customer cars. A thermostat fixed every one of them.

One in particular was a 2008 Aura. It came in with no A/C, DIC reading dashes and a P0128 set current but no SES light. I put a thermostat in it, cleared the code and never saw the car again.

The thermostats were redesigned recently from GM. The factory stat has a rubber seal around the valve that will deform and hang it up. The new design part eliminates the seal.

 

If your thermostat was bad, wouldn't the DIC still tell you the coolant temp? I mean.. even when its 10 degrees here in the winter when I first start my car, it will tell me the coolant temp is 10 degrees.
And aside from that, the ECT and thermostat would throw the same code.

 

FYI Malaclypse is right i checked codes and coolant thermostat came up as one code, had thermostat replaced and problem solved

 

In the process of changing the stat your disturbing the wiring to the ECT which is likely your fault. That doesn't mean it's shotgun sure. So below is how I check it.

If I saw the issue on a customer car, first thing to look at is what the reported ECT is. if it's -40 then the problem IS NOT a thermostat. To confirm I would actually get in there and wiggle test the wiring at and near the sensor. By changing the thermostat as I stated though unrelated in the process you have likely disturbed the actual problem and caused it to function again. Most of the time if you never see a customer car again for the same problem down the road it's because they took their business elsewhere. I don't have customers come back because I will verify the wiring and sensor are good before proceeding with a mechanical failure unless it's smack in the face obvious.

However, in cases where coolant temp reads lower than the thermostat open point by a good margin, then Mal would be correct. What trips me up is the --- on the dashboard coolant temp which on my car reads whatever the actual temp is even down into the 20s which is as cold as it gets around here. Ah well, to each their own.

 

I had this exact problem about a month ago, as soon as the coolant temp got up to about 160 ish it would go to ---F, I had to replace the thermostat because terrible dex cool is corrosive and it will corrode the gasket in your thermostat. Just a heads up in case it comes back. Also I didn't think that it did anything to my A/C but during that time my girl was driving my car and she came in saying my A/C wasn't working and it was blowing warm air. I then went out to check and upon restarting it the A/C was working again. Maybe just a coincidence though, who knows.

 

this just came up on my car this morning also. coolant reads ---F and i checked the code and its throwng p0128. sry, confused on wat ya'll were saying...do i need to replace the thermostat?

 

Yes, you need to replace the thermostat. That will take care of it.

 

mine did this samething over a year ago. its nevercome back and i havent fixed naything in the cooling/AC System

 

here we go again. Read this thread and make your own decision

 

Yes replace T-Stat, if you have access to a scan tool that can look at your ECT Sensor temp, be sure to verify it as working before you do so. Apparently part of the P0128 code is that it changes the tables on the car's ecu in which part of it disables reported coolant temp (the --- on the display) and turns off the air conditioning(body control module function) which is simply a direct result of lack of reported input (any temp not reported or below 32F). It appears this is a incorrect programming issue from GM and several other manufacturers who use the same series of ECU. On fords, reporting the P0128 it does NOT shut off the a/c nor does it zero out the reported temp. It also sets the SES light. I dealt with the exact problem twice this week on a 07 F150 and 06F150 respectively. Malaclypse, sorry man, just never assumed the ECU command to disable reporting.

 

my cobalt SS is doing the same thing. A/C stops working and coolant read out says --- . After it cools down and warms back up everything starts working again. Went to the dealer they said no codes..was workin fine when i took it in. Seemslike it does it more when its cooler outside. Thought about changing tht temp sensor. Does anyone now where it is located? Any more info would be great,,,thanks

 

Its no big deal, man.. believe me I wouldn't have believed it either if it hadn't happened to both of my previous Cobalts. In addition to other 2.4 cars I've worked on.

I didn't get all butt hurt about it. =)

Silver Cobalt SS: The dealer can't realy condemn a part without a code. I'd bet the thermostat is the trick more than the ECT. Its just getting the code to set and the problem to occour when its with them.

 

changed my t-stat and everything is fine.now....had a po128 or po126 what ever it was code. change your t-stat and everything will work normal.

 

Wait
this makes no sence to me.
I'm not saying what you said is untrue,
I just don't understand it.

When I go out to my car in the morning and its cold and I turn the ignition to on and engine off,
The coolant temp reads something like 60F or whatever the temp is.
When I start it, the temp numbers climb.

1: Why do I NOT get P0128 appearing then disapearing once tempature gets up to temp.
2: How does the temp sensor know that the thermostate is stuck open to show a ---F instead of just a cold engine to show 60F or 64F ect?