99 Grand Am throwing out code P0118 [Archive] - GrandAmGT.com Forum


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10-18-2012, 01:51 PM
Hi everybody,

First post here. After lurking this forum for a while, I decided to join the community here. You guys are the nearest thing to Grand Am experts I could find on the internet. Hopefully, somebody can provide a bit of help.

I have a 1999 Pontiac Grand Am SE that has been acting up. The temperature gauge is not currently working. The indicator sits at the lowest end of the temperature range. The Service Engine light prompted me to check with a scan tool and code P0118 repeatedly pops up.

In an attempt to solve the problem, I have replaced the engine coolant temperature sensor and the thermostat. Neither of these helped the situation. Could the wiring possibly be bad?

A couple other notes:

The radiator fans kick on almost immediately after start up and the AC doesn't blow cold air. (Before this problem started, the AC ran fine.) Could this be some kind of fail safe related to malfunctioning sensor wires? My guess is that it is better to have the engine too cool than too hot, in the case of a sensor or wire failure.

The car sometimes has trouble starting up and when it does, the idle is pretty rough.

Any and all help is appreciated.

10-18-2012, 01:58 PM

when i had this exact problem i changed both the coolant temp sensor, and thermostat and my problem was fixed. idk why you are still having the problem, maybe you got a faulty part?

A code P0118 may mean that one or more of the following has happened:

A bad connection at the sensor
An open in the ground circuit between the ECT sensor and the PCM
A short in the voltage feed between the sensor and the PCM
A bad PCM (less likely)
A bad temperature sensor (shorted internally)

10-18-2012, 02:04 PM
Yea, it's a V6. I was hoping that neither the ECT sensor or the thermostat were duds. Especially the thermostat, because replacing that was a pain in the butt.

What do you think, should I try swapping out the ECT sensor again to see if it makes a difference?

10-18-2012, 02:23 PM
yeah the thermostat is a pain. umm... i would say ect first since its easier, but also dont want you to waste money and it not fix the problem... any other lights on the dash on? and hopefully someone else wil pop in here and give some insight too.

10-18-2012, 02:33 PM
The only other that thing that has been coming on is the low coolant light, but I think that is an unrelated problem. I've been battling a couple different coolant leaks.

When you replaced your ECT sensor, did you seal the threads? If so, what did you use?

10-18-2012, 03:31 PM
boom. replace your coolant resivoir. the sensor in it is bad. as long as its at the "full" mark.

after you do that, tell me if the problem is still there.

10-18-2012, 05:53 PM
I thought that the sensor in the coolant reservoir only communicates the level of coolant. I didn't think it had anything to do with the temp gauge.

I was planning on replacing the reservoir anyway though, because I think it is screwing up the rest of the cooling system (not holding pressure/causing air bubbles in the system).

Anybody know which fuses are associated with the ECT sensor? I haven't been able to figure that out when checking them.

10-18-2012, 09:08 PM
i cant remember but i think it made my fans go when that res sensor went bad. doesnt hurt to replace it and see and seeing that it does need replaced.

idk about the fuses that run to what.

10-18-2012, 11:38 PM
How do you follow the wiring coming off the ECT sensor? It seems to disappear into the top of the engine. I'm dreading having to tear everything apart just to find the sensor wire.

To be sure, there is only one sensor related to coolant temperature in this model, right? I've seen some conflicting info that there are actually two sensors -- one for the gauge and one for the PCM. The Haynes manual is totally worthless in figuring this out.

10-19-2012, 12:46 AM
umm im not sure, never had to... why are you trying to follow the wires?

10-19-2012, 12:08 PM
I'm trying to determine if there is a short in the voltage feed between the sensor and the PCM.

10-19-2012, 01:35 PM
oh, wish i could help on that. :(