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Replace your temp sensor to start... That will fix the immediate peg of 120*c and also will then tell you what the actual temp of the vehicle is when running to let you know what operating temp it is reaching.

That, or the t-stat needs replacing or you are low on coolant.

I guess I looked too much into his original post that says his thermostat is working...

We need more info to trouble shoot this issue...

How quick does it peg to 120*C?

How do you know the car is not to operating temp?

Ok, so your saying the pipe is not hot and the temp is pegged? Or the pipe is hot and the temp is still pegged?

I would start at checking fluid level and then if you have as ace suggested try changing the thermostat, your original post read as if the temp shot up almost right away with no warm up time.

I can't answer that question, I don't and never did own a OBDI car capable of that function, but some cars go into a diagnostic mode which will sometimes trigger the fan to come on.

your not supposed to run diag with the car running only use while key is on engine off. But yes the fan should run when you do the paper clip trick/code reading means its working right, did you get any codes? you have to remember to count the flashes and it continues the pattern 3 times. 1 flash pause 2 flashes is code 12 which means there are no codes stored.

Get the AC Delco's if you can.

spacecadetz26 wrote:......1 flash pause 2 flashes is code 12 which means there are no codes stored.

This is incorrect. Code 12 simply means the diagnostic system is functioning as it should. Nothing else. Period. Yes, the fan should kick on almost immediately upon jumping the A and B terminals. Your situation sounds most likely to be a bad/stuck T-stat as suggested or a faulty CTS (Coolant Temp Sender, not the Coolant Temp Sensor). I would start with checking your coolant though as suggested. Make sure it's at the proper level cold and doesn't look like a milk shake.

Btw, Standard anything sucks ass. Get the AC Delco in this case.

i think steph was just saying that as a general... yes it means the system 'is working'... how good of a system gm had? i cant attest too. i know i had a simple misfire code when i needed a new ICM... where as the ford system (in 1993-95 at least for sure i know) had many more codes that pointed in much less vague directions.

anyways, yes first things first, check your coolant and oil levels and color/texture. and try to describe the issue a bit better.

is you temperature gauge pegged immediately to hot upon cold start up? if not, and the gauge runs up in that direction within minutes, then you likely have a cooling system issue. lack of coolant (look for leaks, and or extra steamy and sweet exhaust, milky oil), could be stuck thermostat not allowing coolant to flow out of the engine through the rad. could be a clog somewhere (really old coolant, chunks/gunk, possibly 'temp/stop leak' fixes).