The code P0128 will set if the engine hasn't reached operating temperature within ten minutes of it running. Usually these ecotecs will reach full operating temp in 5 minutes or less. THat's how the PCM knows to set the code.

 

its because the t stat is stuck open and the car isnt warming up in time for the computer so, it sets the code...my car warms up real quick now....and runs hell lot better...trust me its very worth putting in a t-stat..not a sensor....

the rubber gasket was holding the t-stat from closing..sure looked like it....

 

Ok cool thanks


So what about the other part?
How does a malfunctioning thermostadt cause the display to read ---F?

When the temp display can show regular outside tempature when the engine isnt even running and the coolant is the same temp as outside? BUt then when the thromostadt goes bad ,and is stuck open, it knows to display ----?

 

Read the part in bold and the part just after it.

He explained it better than I could.

 

this just happened to me i replaced the thermostat and am still getting the code even after disconnecting the battery for 15mins then driving the car.. but a/c blows cold and everything works fine.. now here is the quesiton i just had the car tuned last week with a trifecta tune and this is when my problem started could the tune be messing up my car? to throw this code bc it literally happened within 15mins of getting tuned last thrusday

 

Its possible. What're your fan turn-on temps looking like? Also did you put a lower degree opening thermostat in the car?

 

i put a 180 degree thermostat in the car.. whats funny if im stopped at a light my temps never go above a 100 and i live in florida so its not even getting to full operating temp.. but if i turn off the car and turn it back on my temps jumper like 30 degrees.. and all this has been happening after i was flashed with the trifecta tune.. my fans havent even kicked on so idk what i should try next... any suggestions would b great

 

I'd put the factory thermostat back in. I believe the operating temp for it is 195. That could very easily make up the difference and set that code for you.

Its really better to work closer to stock coolant temps, anyways. Considering the engine tolerances were designed around that opeating temp.

 

I submitted a technical inquiry to GM referencing the WHY the P0128 disables A/C and temperature display. I believe that so long as ambient temp is above 32F you should maintain command of the A/C, it's not as if it matters, the calibration compensates for engine temp anyways, the code is merely to tell a tech the Thermostat bypass is locked open or it's stuck (on some engines a thermostat has what's called an emergency overtemp bypass, where it will lock open and stay open even after the engine cools off.

 

thats the kicker i had the factory thermostat in the car when i started throwing the code.. so idk if the tune messed up the car or what.. idk if it matters also i have a 2006 cobalt and when my motor blew the shop ended up putting a 2005 motor in the car?? and ive read online that their is a tsb for the 2005 models could that be causing the issue im going nuts trying to figure this out

 

Ok so I've had this exact same problem with my Colbalt SS amongst many others...and I have replace 2 thermostats in 3 months and my car is now reading --- once again... Has to be something else? It turns on and off and when the coolant reading is --- the fan runs super high, I brought it into the dealership and they said my engine is running cold and I need a new thermostat, AGAIN?

 

I had one case of a bad ECT sensor setting a thermostat code.

All the rest have been thermostats.

 

I'm quite certain my T-stat is stuck open. even on 95* days my coolant wont go above 160s. on most days it stays in the 140-150 range. It used to reach 190s on hot days or while idling and usually stay in the 180s while cruising.
I have no CEL and my A/C works just fine. :-\

 

another question... When buying a new t-stat, do we have to order the whole housing to get the thermostat? I dont see a part # listing for just the thermostat, only the housing.

 

You can get the thermostat separate. I don't know the number and I'm too lazy to look it up on CED.

Just don't get one of those aftermarket 180 or 160 degree thermostats. There's no point.

 

yeah, i was just going for stock replacement. no need to screw with it.
If it's on CED i'll find it, i didnt know they had thermostats.

 

the thermostat I got from auto zone looked more durable then the gm one. FYI I removed the fuse box when I replaced my thermostat cuz I had a hard time gettting to one of the bolts. Remove the fuse box and make your life easier.

 

In for results. If you are getting a blank on the temp display you have an open in the ECT circut somewhere.

Thermostats are a PITA on these cars with the motor installed, but it can be done. I would suggest, while you have it out, drill a small hole in the new one, on the top side. Lets the system bleed itself much easier the first time, no waiting for the thermostat to open up to push most of the air out of the coolant passages.

 

Just had this happen to me driving downt he highway my AC shut off and my coolant reads "---" heat still works, and radiator is touchable... if its the ECT sensor, where would i be recommended to go? i'm not under warranty, so i'm sure the dealer would be quick to take my money....

Thanks

 

Buy the sensor and change it yourself. Be sure it's the sensor. My car had an issue with the wiring harness rubbing though where it passed under the manifold, causing an intermittent short, which would spike the coolant temps to max. Wrapped the section of harness and the problem never came back.

I have a couple used ECT sensors, i'd be happy to sell ya one cheap to try and see if that's all you need.

 

thanks i'm going to see what it is then let you know for sure!

thanks again! too hot in wisco to not have a/c haha

 

99% of the time the "---" readout isnt due to the sensor but due to the failed thermostat. When the ECM detects a failed thermostat it disable the A/C because it doesnt want to have any issues controlling coolant temperature. A/C adds engine load and reduces radiator efficiency make it harder to regulate temp, its just taking precautions to avoid overheating.

 

If the t-stat is stuck open, the temp display will just show cold. The --- on the display is a failure of something in the electronic system.

 

Nah, respectfully Joe, I see it all the time. Even on first setting of P0128 the display goes to dashes, and depending on vehicle it will disable A/C and turn on high cooling fans

Most common issue with the tstat that causes the issue, is the rubber inner seal swelling/separating from the assembly and jamming the thermostat. New style tstats have omitted this seal, and are back to a metal seal.

A signal wire chafed to the engine would short to ground, and result in a a potentially high temp, or an unchanging ECT reading. Again since it now determines that the tstat may be failed or that the sensor is shorted it disables the readout, the AC and turns on the fans.

remember the P0128 is actually an ECT circuit diagnostic,but if the coolant emp isnt properly regulated the ECM cant tell if its a mechanical or electrical fault,and it as soon as it fails the diagnostic it assumes the data is in error and just goes to default settings.

 

Again, what does just a low operating temp indicate? Maven, you're saying that my t-stat is not bad since its not blanking out the ect reading or throwing the cel? The car is showing no indication of anything being wrong other than the temp is 20 - 30 degrees colder than it used to be.