What type of sensor could be malfunctioning if I have to drive with either the heat or air conditioning on? Otherwise the engine will overheat.

Sounds like a low flow problem. How old is your water pump? Maybe a blockage, but you replaced the radiator, hoses and flushed the system already, right?

To your knowledge was any stop leak used in the cooling system, if there was it may have plugged up some of the smaller coolant passages. If you did do a flush was it a back flush?

So here's what got me into this mess in the first place..... transmission cooler was leaking fluid into the cooling system. At that time the coolant was pretty nasty! Rad was gummed up pretty good, so i figured it would be easier to replace it. Flush upon flush, (back flush included) conditioners, cleansers used to get the system looking good again. Thermostat seems to work well. After all those flushes I figured I had the system clean. The waterpump is oe.150000Km on the car, guess a waterpump wouldn't hurt eh. It just seems weird that it won't overheat when the a/c is on. One would think that would cause the engine temp to go up. Thanks for the suggestions guys :)

You might check the engine cooling fan and make sure it is turning on when it is suppose too. If it is not working that would explane the overheat without the A/C turned on, when you turn on the A/C or defrost it turns on the condenser fan out front. Make any sense?

Makes complete sense now, thanks for your input. I never did check that. I know the fan's come on when the heat or a/c is on. So if it turns out that they don't come on, would it be reasonable to guess that the temp sensor is faulty?? Not sending the signal to turn the fan's on?

That is a reasonable assumption. Don't forget to check the relays also.

How long have u owns it ?

I bought it with 25000 km. Problem is still there. I've checked the fans, they come on when they should. With the crazy weather here in Calgary, it's not to much a problem driving around with heat or a/c, but I really should get this issue resolved.