Hey, folks, I need some help. Today while driving home the coolant gauge in my '14 dropped to 0, and the cooling fan came on full speed. Switching to the Tour panel display in Performance mode showed 0 degrees engine coolant temperature. Everything else is normal, including cabin heat. Coolant level is correct. On shutdown the fan continued to run, but I was able to shut it down by starting the engine (which apparently shuts off everything not related to starting the engine), then immediately shutting down.

ETA: I then cleared the codes (engine off), restarted, and the codes immediately reasserted themselves.

I went digging through the CF archives and found threads that discussed the temp sensor mounted in the water pump assembly. Pulled the connector and verified the temp sensor is reading in the correct range (around 3k ohms) for a relatively cooled down engine. Plugged the sensor back in, started the car up. Same symptoms. Plugged in my trusty Blue Driver diagnostic tool and pulled 2 pending codes:

P0119 Engine Coolant Temp Sensor 1 intermittent
P0129 Coolant Thermostat (Coolant temperature below thermostat regulating temperature)

I pulled up live data on the scanner and it showed the coolant temperature slowly warming up; all the while the cooling fan is screaming and the instrument cluster is showing "0" degrees.

Is there a 2nd sensor somewhere that feeds the instrument cluster and fan controller?

Thanks for any help, and have a good one,
Mike

 

One guy noted that you may need to clear the error codes with a scanner or by disconnecting the negative battery cable for 30 min (you will need to reindex the windows). I have only heard of the coolant temp sensor on the water pump manifold.

Ron

 

Thanks. Sorry, I should have mentioned I did clear the codes, which came back immediately. I'll update my original post.

Thanks again, and have a good one,
Mike

 

I just went through this. I too had zero on the needle and analog performance screen, and a screaming cooling fan from startup on a cold motor. I did not have a check engine light and did not scan for codes. I did change the sensor (though I did measure the resistance and got about 3.4k at around 70 degrees. Just changing the sensor did not cure the problem I had to disconnect the battery for a few minutes and that did seem to clear the problem. I've been driving it for the last few days and so far all is good. I'd disconnect the battery, press the brake pedal and wait a few until reconnecting. You will need to reindex the windows but thats about it. Be careful letting the fans run until they shut off, this pretty much killed the battery.

 

Clearing the DTCs should do the same as disconnecting the battery. And absolutely just letting the fans run; I stumbled onto being able to shut them down by starting the engine and immediately shutting back down. Otherwise they run for 18 minutes before they shut down.

When I get home from work today I'm going to do a harness check (a.k.a. the "wiggle and watch" test). I'll let y'all know what I find.

Have a good one,
Mike

 

UPDATE: After work today I fired up the scan tool, monitored the coolant temp sensor, and cleared the codes, again. Monitored the temp value, and verified the sensor was reading ambient temperature. Instrument panel still read "0" degrees. Started up, same symptoms as before.

So, I tried something different. I cleared all the codes in all the car's systems, and that fixed the problem! Unfortunately I didn't read all the codes before clearing them, so I don't know which DTC was causing the problem. My guess is the instrument panel computer, but that's just a guess.

Oh, and now we understand why disconnecting the battery works so well; the DTCs from whatever system affects the instrument panel and cooling fans are cleared as well.

If I learned anything, it's to follow the data. Everything was telling me that the local wiring harness and the temp sensor were OK, and that turned out to be the case.

Y'all have a good one,
Mike

 

Thanks for the update and confirming why the default answer to many a C7 question is - disconnect the battery.

 

Odds are it's your thermostat. Either you're running a lower temp thermostat which is opening quicker than stock (causing the engine to not come up to temp as fast as the ECM is expecting), or your stock thermostat is failing open.

 

Read post #6.

 

OP glad to see you got it fixed but in this car it is almost crazy it one set of wires are it goes to something else. The only true way I have seen to clear every thing the easy way is to unhook the NEG batt cable for sure for about 30 mins. Then if something comes back you do have a problem with something & just have to run it down. Robert

 

My Blue Lemur OBD-2 code reader was able to clear the needed codes. I haven't needed to disconnect a battery cable since I bought it.

 

Good luck You fixed the symptoms, not the problem. It'll come back.

 

True. I don't know what the trip was, but somewhere some system (instrument panel controller?) latched a failure code that would sympathetically trip the P-codes on startup. Clearing the P-codes had no lasting effect at all; they would immediately re-assert. Once I cleared all the codes everything has returned to normal ops. The onset of the problem was sudden and total, so as you say, something is out there.

To your point about the thermostat, it's stock. It's also 8 years and 130k miles old. I've been watching it, and the engine warmup and temp regulation characteristics haven't changed. But like you said, there's a problem out there, somewhere.

Have a good one,
Mike

 

So, after about 8 months and 10k miles later, the cooling fan/instrument coolant temp issue raised its head again. This time I didn't clear anything, but rather got with the head auto shop instructor at our local community club (don't even think about our local Chevy service dep't.), who spent a good half hour going through everything (his students were in attendance). What we learned is what was discussed upthread, that the problem was with the thermostat.

Turns out that the PCM monitors the coolant temperature rise as the engine warms up, and if it doesn't fit a predefined profile (adjusted by various and sundry sensor readings), code P0129 will be set indicating the thermostat has failed open, which turns on the cooling fan full speed and sets the coolant gauge to zero to alert the driver.

Now, why the fan won't turn off when the engine is shut down we couldn't figure out.

Y'all have a good one, and a Happy Thanksgiving!
Mike

 

With your cooling fans remaining on are you still seeing this P0119 ??…the P0119 is a coolant temp issue and the circuit has a 5 volt reference and a ground…the ECT sensor pulls that 5 volt reference to ground through the sensor and depending on the voltage will display your coolant temp…if that 5 volt reference is shorted to ground the ECT will max out on your gauge…if you remove the ECT connector and jumper those 2 wires the cooling fans will come on because the ECM is seeing that high temp….the ECT sensor may be defective or the 5 volt ref wire is actually shorted to ground somewhere…remove the ECT connector and see if the fans stop….the ECT sensor is a thermistor type circuit and the only type you can short like that without causing damage to the engine computer.

 

I see you have a scan tool so can your scan tool display generic or global OBD2 data ??…in Global OBD2 the ECM must display the actual ECT temperature and not a “substituted value” as we call it…your gauge may say 180 degrees but in Global it may be reading 280 degrees and your fans come on…a bad AC pressure sensor may also cause the cooling fans to stay running.

 

Code P0129, not 0119.

Temp sensor was tested as documented in post #1, as well as other symptoms documented.

 

You say you had a pending P0119…there is wiring that makes the sensor operate so just testing the sensor you haven’t checked everything !!…like I said that 5 volt reference gets shorted to ground the fans will come on.

 

Oh, okay, that was back in February when I disconnected the temp sensor to verify via OBD2 that the wiring wasn't shorting out somewhere intermittently. Wiggle tests, etc. Once I cleared that code it never came back.

My update was to close the loop on what's happened recently. I guess it took the onset of cold weather to get conditions right for setting the P0129 code again, and the subsequent digging we did to figure out exactly why it was being set. If it comes back again with the new t'stat installed I'll let y'all know.

 

This happens every Fall. Temps come down, and the Vette goes crazy, ECM coolant temp readings and fan settings indicate a cooling temp problem. People immediately replace the coolant temp sensor, and as they do that they reset the system, and symptoms go away....till next time.

It's not the sensor, it's not the wiring, it's the thermostat.

If the coolant temp doesn't increase as quickly as the ECM expects it sets the P0129 code, turns the fan on high, and makes the coolant temp gauge not operable.

Most likely it's an aftermarket, lower temp thermostat, or a failing stock thermostat.