Engine cooling fans run continuosly

I recently changed the thermostat on my 1995 Bonneville and I noticed that the engine cooling fans are running continuosly. Could the coolant temperature sensor be malfunctioning?

Welcome to AF! There are two possibilities here. The temp sensor and the fan relay. Sometimes the relay will go bad and the fans will operate continuiously. The other is that the temp sensor sends the wrong temp to the PCM and it turns the fan on. I would change the sensor first and then the relay if the sensor doesn't work. Unfortunately I an not exactly sure where the relay is on the Bonneville. Check in the power control center under the hood but I am not sure if that is where you will find it.

Another possibility is air in the cooling system. If the coolant sensor isn't submerged in coolant, it can send a false reading that the car is overheating, kicking on the fans.

Another possibility is air in the cooling system. If the coolant sensor isn't submerged in coolant, it can send a false reading that the car is overheating, kicking on the fans.

I bled all the air out of the system yesterday. I'll install the new "$7.00" coolant sensor and see if that solves my problem.

I bled all the air out of the system yesterday. I'll install the new "$7.00" coolant sensor and see if that solves my problem.

Installed a new Coolant Temperature Sensor switch located just below the thermostat housing. Both cooling fans still run continously. Engine temperature fluctuates between 195-215 degrees.

Installed a new Coolant Temperature Sensor switch located just below the thermostat housing. Both cooling fans still run continously. Engine temperature fluctuates between 195-215 degrees.

Went out to start my Bonneville last night and it BACKFIRED! I kept trying to start it but it would just run for a couple of seconds and die.

Any ideas as to what's going on guys?

I bet your relay(s) is/are stuck shut. They is located in the relay center along the firewall in the engine compartment. Look under the relay center cover and there should be a diagram telling you which relays are which.

I bet your relay(s) is/are stuck shut. They is located in the relay center along the firewall in the engine compartment. Look under the relay center cover and there should be a diagram telling you which relays are which.


I did looked under the relay center cover this past Saturday but I did not see any diagram. It could have fell out when I had my evaporator core was replaced last year. How difficult is it to change the relays? Is this a dealer item/repair?


I'm sorry guys, but I believe I may have given you the wrong information. My cooling fans do run continuosly, but only when my inignition switch is in the "accessory" or "on" position.

On my 1998, the fans would always run when the A/C was on. Nobody ever believe me when I said the engine ran COOLER with the a/c on ;)
Now, with my 2000, this is not the case. It works just like you'd expect it to.

On my 1998, the fans would always run when the A/C was on. Nobody ever believe me when I said the engine ran COOLER with the a/c on ;)
Now, with my 2000, this is not the case. It works just like you'd expect it to.


I located my backfire/stalling problem. A vacuum "T" located at the back of the plastic intake manifold plenum, had partially came out. I pushed the "T" back into the plenum and it cranked right up.

I must have bumped the "T" when I installed the engine cover.

I located my backfire/stalling problem. A vacuum "T" located at the back of the plastic intake manifold plenum, had partially came out. I pushed the "T" back into the plenum and it cranked right up.

I must have bumped the "T" when I installed the engine cover.

I disconnected the A/C High pressure switch but the fans continue to run whenever I turn the ingition switch is on.
Is there anything else I need to check? I thought that the only time the engine cooling fans engage is when the engine reaches a specific temperature or the A/C is on.

The relay's are a little box that looks like a big fuse, only they have 4 or 5 prongs. It's a simple job to swap them out. You unplug the old one, pull it from it's mount and replace it with a new one. The relays should be under a plastic cover just above the heater blower motor on the firewall in the engine compartment. They are about 1" square and should have the number 12193601 on them. By the diagram in the GM computer, it appears they will be mounted in the 3rd and 4th positions from the drivers side.

The relay's are a little box that looks like a big fuse, only they have 4 or 5 prongs. It's a simple job to swap them out. You unplug the old one, pull it from it's mount and replace it with a new one. The relays should be under a plastic cover just above the heater blower motor on the firewall in the engine compartment. They are about 1" square and should have the number 12193601 on them. By the diagram in the GM computer, it appears they will be mounted in the 3rd and 4th positions from the drivers side.

Thanks Rich!

Scott, can you tell if the cooling fans are running at high speed? If and when you do locate the relay/relays, when you remove relay, start engine to verify cooling fans remain off. Problem doesn't have to be the relay either. PCM commands relay to pick up, and drop out as required based on ECT sensor input.

I replaced the relay and the coolant temperature switch but both fans continue to run at high speed as soon as I turn the ignition switch. Would replacing the coolant temperature sensor solve the problem?