Need to know which temp sensor turns on the cooling fans,there are two sensors at the rear of the engine next to the coolant return hose, one is a two wire , the other is one wire. I assume one is for the dash gauge

You still have many other components in that system that has to be in working order for the cooling fan to work properly. The cooling fan is turned on when engine temp reaches a predetermined temp, usually around 220-230 degrees F. the ECT (engine coolant temp) sensor siganls the ECM (Engine Control Module) to ground the cooling fan relay or cooling fan module(if equipped) and if the cooling fan fuse is good and the fan has power to it the realy will complete the ground for the fan to run. You may need to have a qualified technician diagnose the problem if the obvious things like fuses and good grounds are not the problem.

Hope this helps.

This sounds like a bad coolant temp sensor that give computer false reading, check the coolant temp sensor connector and if it is good with no bad wires to connector then replace coolant temp sensor , when the computer gets a signal from temp sensor that engine is hot it turns fans on high speed to cool radiator and displays a/c turned off , if this truck does not have electric cooling fans then they wont run just explaining what happens depending on engine and cooling set up. replace coolant temp sensor and check connector.

Three thing you need to be concerned when the cooling fan fails to turn on, the fan, the fan relay, the coolant temp sensor, First the relay,go to the engine junction fuse and relay box there listed next to thethe ABS relay heater blower relay is the cooling fan relay, Now make a tool from the cooling fan relay,take the cover apart and expose the relay,when you collapse this relay the fan turns on stick in back in the socket key on engine off collapse the relay the fan should turn on if the fan doesnt turn on a fan is the solution if the fan turns on the coolant temp sensor will be the solution this sensor is located at the thermostat housing just follow the top radiator hose,you will see a sensor there

This coolant temp sensor is located at the top right rear of the engine. If you are standing in the front with hood open it will be to your right at rear of the engine.
Depending on if the car is supercharged or not the sensor is in the same area, the only thing between the two is if its super charged you will need to remove the air inlet and turbo hoses to gain room to get to sensor.
This is easy to replace, just unplug the connector and then using a socket remove the sensor. Note: this should be done with a cold engine , and a pan will be needed under the car to caught coolant that will flow out.
Keep radiator cap on to limit the amount of coolant that runs out when sensor is removed. have new sensor ready to install as quick as you can to avoid lots of coolant loss.
When done, fill coolant as needed. good day.

Sounds like you have a faulty head gasket or maybe a cracked head.This can cause overheating and the check engine light to come on.The code for the o2 sensor is probably caused by coolant getting into the exhaust system and coating the sensor.The coolant fan may work but it cannot remove enough heat from the rad to make the engine cool off.Try some engine coolant stop leak to see if it will help.This maybe only a temperary fix.Let me know if you need more.MOE

I had this problem with a 2007 Chevy Cobalt. I changed the coolant sensor and even though the check engine light went out I still had a reading of "---" on the coolant gauge. The problem is most likely one of three things: a low coolant level, a bad coolant sensor, or the thermostat needs to be replaced. The only other possible thing it could be is a bad BCM (Body Control Module) which in that case you would most likely be having other weird problems.

The Thermostat and the sensor are both very easy to get to and replace. Take off the engine cover by removing the oil cap and they are both to the rear of the engine on the driver's side. They are right next to each other.

Hope this helps!

V6 2.8L top right rear of engine. Fan also turns on through A/C pressure switch and coolant temperature sensor, but these should operate independent of the coolant temp switch.
V8 5.0L and 5.7L (Heavy Duty) lower right rear of engine above starter.
Base model V8 uses the coolant sensor (top right of engine) and A/C pressure switches (lower right corner of eng compartment on A/C line to turn on the cooling fan. Fan is energized on all applications through the cooling fan relay.

I take it this only happens when the vehicle is stopped & idling? If not, check radiator & hoses for blockage (since you've already replaced the thermostat & top hose) - repair/replace as needed. Next, check the coolant temperature sensor's operation (there are two sensors - one for the temp gauge on the instrument panel, the other is used by the ECM (PCM). If that one is malfunctioning (i.e., telling the computer that the engine coolant temperature is less than it actually is, The ECM (PCM) it won't turn on the cooling fans.
Since you didn't specify which engine you have (i.e., 2.4, 2.6 or 3.5), Photo locations of all three are included here.


Hope this helps.

The Coolant temp Sensor is located right under the top radiator hose (on the engine side), with a little black connector!, You might have to remove th etop hose to get to it with a deep socket and extension!.
Also Dont Forget to blled the coolant system by the bleeder valve uptop right near/above the coolant sensor!.(10MM Mini socket w/ extension IIRC!)

Cooling Fan Temp Sensor is the First thing You Check (with what you have described) Cooling Fan Should ALways come On When The A/C is turned On as A/C Adds A Load to Engine, If Temp Sensor is Faulty it Will Not Give GROUND Signal to ECM >For COOLING FAN >RELAY to Give ground and turn ON The Fan.
There ARE 2 temp Sensors that Serve Different Purposes 1 is for Dash READING GAGE & Or TEMP LIGHT the Other is For ECM or Fan Relay (SAME Thing) you Said>""can disconnect the sensor and fan continues to run" < This Leads me to think you Changed the Wrong One. Not Questioning your Skills Just Here to Help.
TEMP SENSOR FOR 3.1L V6 is Located upper RH rear of engine compartment, near exhaust system. Temp SENSOR for 2.3L Engine is Located in LH rear of engine compartment.
I Hope this Helps Lead you to a NON-OVERHEATING Pontiac.
Good Luck and Have A Great Evening. rejakwilson