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My sister has a 1995 Chevy Lumina with the 3.1 liter engine. Her car tends to overheat at idle; we have determined that the radiator cooling fan does not kick on.

In our efforts to diagnose the problem, one possible solution appear to be a faulty ECT sensor. However, the dash gauge does work (that is how she determined it was overheating in the first place); as such, I am curious if this render a problem with the ECT sensor unlikely? That is, is the ECT sensor responsible for sending information to both the dashboard gauge and the coolant fans, or just the coolant fans? If the ECT sensor only controls the cooling fans, how does the vehicle monitor operating temperatures and send this data to the dashboard gauge?

Thanks.

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    I'm not sure on your Lumina, but Chevys around that time used two temp sensors, one for the PCM and a separate one to register on the dash. The PCM controls the fans. It could be the one for the PCM is bad. You may be able to figure this out by plugging in a code reader to see what the PCM is saying the temp is and judging against the dash.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented May 20, 2017 at 23:50
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    If the car is warmed up and starting to overheat at idle, see if the fans engage when you turn AC on. If not, an electrical issue is more likely, such as the fan relay. Hopefully this gives you a good starting place.
    – Mustangguy809
    Commented May 21, 2017 at 0:09

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The answer to your question is more simple than you may have thought. There are actually two separate sensors for this model. One is a sending unit that sends info to the gauge, not the ECU, which means that it's also possible for your gauge to give an incorrect reading or fool you into thinking the car is overheating when that's not the case at all and there's nothing wrong with the car at all (except for that sending unit being defective). The other is the ECT sensor that sends coolant temp info to the ECU in order to keep everything reacting appropriately. So yes, there could be a problem with the ECT sensor even if the gauge is operating properly.

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