Silverado overheating

I have a problem with my truck overheating if it sits at an idle or very slow speeds (5-10mph) for too long. I originally had problems with the Dash information center giving me a warning that the truck was overheating and AC was shut off, and the temp gauge reading flat or not registering at all. Did some research and found this may be contributed to a bad thermostat not letting the truck reach operating temperature. Changed the thermostat and knock on wood, no more errors. I still have the problem of overheating at an idle. I tried changing the temperature control sensor on the side of the block, but that did not resolve my issue. I have checked and found the fan coming on without turning the AC on, but now it doesn’t seem to want to do that. Once I turn the AC on, both fans come on and the temp gauge goes back down to normal. Do I have a bad relay or is the fan messed up and need to be replaced?

If both fans are working, the fans are good. The problem is probably a bad temperature sensor that’s not reporting an accurate coolant temperature.

Or it could be a bad relay. Swap out the relay with another one from the box. See if that fixes it.

Check for a second temperature sensor. Often the temp sensor to control the fans is not the same one that feeds info to the computer and the dash. It may be near the bottom of one of the tanks on the side of the radiator.


If it was a relay, the fans wouldn’t turn on with A.C.

make sure the system is bled correctly. it could be something as simple as a air pocket

There is not temp sensor on the tanks. I just replaced the one on the block. I’ll check the relay tomorrow in the daylight.

Usually the real mechanics pop in before now, but looking for bubbles after removing the radiator cap when cool has been suggested as a possible head gasket leak. Also trans overheating can stress the cooling system. How does the tranny fluid look? Also a pressure test or maybe even just a new radiator cap is in order. Sorry to step on the pros toes.

Truck doesn’t have a radiator cap. The “Cap” is on the coolant tank on the side. I’ll check the trans fluid in the morning as well when I have some light.

If your truck is 1999-2014 and both fans come on when you turn on the AC, then all the relays and fan motors are good. If the radiator cap (overflow) cap were bad or any part of the cooling system were losing pressure, the engine would get hot going uphill at highway speeds, not at idle. If the head gasket was breached or the thermostat bad, it would overheat all the time, but would overheat more under load.

So that narrows it down to the ECT sensor in the block, the PCM, or the gauge. If the gauge is working properly, which it seems to be doing, then either it is down to the PCM or ECT.

You replaced the ECT but I’m wondering if you got a bad one or something else is affecting it, making it more sensitive than is should be. Just a wild guess here, but I am wondering if it could be a ground issue. This would be very difficult to find. I don’t think it is the PCM as the PCM is commanding both fans on with the AC.

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Ok, finally got a break to look at things. I swapped the relays and that did not seem to be the problem in that the high/low fan relays (2 different relays) were swapped, turned AC on, and fans came on, so I ruled out the relays. I played around with the connection at the sensor, was gonna get a test light to see if I could see if there was a current going through, but couldn’t get it off (fingers too big to get them small tabs). Then, the fan came on without the AC. It didn’t stay on long, but it came on. But, only once…about 5 or so minutes later, the temperature gauge started rising again and no fan this time until I hit the AC switch. This has me really stumped now. It seems the fan will come on once after a cold start and then that’s it. Once it cools the engine one time, it isn’t trying to hear it again. Any one have a fresh perspective on the problem?

What was the coolant temperature at that point?

I hit the AC switch at about 225 after it held at 210, normal operating temp for the vehicle since I have had it. If I didn’t turn the AC on for the fans, it would have gone higher, as it has in the past.

So it runs fine with the AC on? My WAG your regular fan is not operating but the second fan that kicks on when you turn on the ac is doing the job.

That’s the thing, both fans work with the AC on, and both came on without the AC after the initial cold start to cool it the first time. It didn’t work after that and I had to use the AC to kick the fans on.

So far I agree with @Keith. Sounds like a bad sensor or wiring to sensor. If sensor is confirmed good, you might try taking tests at the PCM from the sensor. Are you using a scanner to read temperatures? But, I think if a scanner reports proper temps the sensor and wiring are good and you can skip testing at PCM. If sensor is good, and relays are good and fans are good and PCM is good, you may have a wiring issue from your PCM to the primary relay circuit that is supposed to activate relay when the temp sensor requests.

Used my Zurich Z8 code reader and pulled a P0117 out even though no check engine light was illuminated. I know the code means low voltage detected at the sensor, so I am looking at bad wiring at the senor then?

It seems . . . to me, who’s been reading this for several days now . . . that the coolant temperature gauge is probably reporting the correct temperature

What engine do you have?

Since you say you have a 1500 Silverado, that means you have the smaller engines

Do you by chance have the 4.3 liter V6 . . . ?!

If so, that would explain a LOT

If you do happen to have that, I’d be surprised if your lower intake gaskets are NOT shot

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I do have the V6

Not sure I understand the correlation between the gasket and the low voltage input code.