Where are the temp sensor located for 91 Regal?

[Friggin]

Hi there... My buddy has a 91 Regal that he thinks he is having temp sensor issues with. His temperature gauge is always reading lower than it should and he is sure it is overheating when the gauge is in the "middle" zone of the gauge instead of up in the "high" end. He says when it gets into the middle area, the "Check Engine" light has come on. It normally runs really low on the gauge... It only gets into the middle zone when he is stopped in traffic etc... Apparently the fan is coming on too - it just may be coming on too late? He has already replaced the thermostat - no change... When this happens he says he usually turns his heater on full boar to help lower the temp in the engine - it seems to work somewhat...

He wonders if I could just replace the two sensors for him (he was told there are two - one for the fan and one for the guage) Is this correct?

If so, where exactly are the two located and are they easy to replace? What are the procedures? Do you have to draing the cooling system or can you simply put a catch basin underneath and replace them, knowing that some coolant will be coming out??

ps.. Also if a garage were to do a test, where would they plug their guage in to actually get teh correct temp reading? Would they plug into the computer or do they do something different?
Any other ideas?

Thanks

[Master01]

What kind of engine does it have??

[Friggin]

Sorry, it has a 3.1

[richtazz]

The sensor for the guage is in the rear cylinder head, and has a single green lead built into it (about 8" long). The one for the computer is in the intake near the thermostat housing and has a two wire connector.