My son has a 2002 firebird 3.8v6. He was leaking coolant at the intake manifold and put some stop leak in it for a temporary fix. The leak stopped and the hasn't had any issues until w few days ago when his temperature gauge stopped working and his fans kicked on high. I hooked up a scanner and he had a couple of codes. I forgot to write them down but one was coolant temp too low for thermostat. The temp was 141 to 145 degrees. I erased the codes and the fan i immediately stopped. The temp gauge started working and it came up to normal. We let it run for 15minutes and I was about to turn it off when we saw steam. It was coming out a seem on the radiator near the top hose. Thinking he needs a new radiator we put a little water back in it and he drove it home. When he got home it wasn't leaking and the temp was normal. Could he have had a hot spot due to the coolant not circulating? I'm puzzled.
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I assume your code was P0128.......

Stop leak could have caused other problems, or not.....

Fact you are adding water/coolant is not a good sign....

When was the last time the system was flushed?

A decision has to be made here.......are you going to fix the leak?

Hey guys, do these do like a lot of cars where if there is no coolant sensed on the temp sender the temp gauge goes all the way high or all the way low?

97BONNIESE, could it be that when the car got home it had leaked so much coolant that there was none to leak out of the radiator as it did earlier?

When I hear "it was leaking" and then later hear "steam", I think coolant got low and head cracked or blown/weak head gasket other shenanigans have occurred due to air pockets in the cooling system and/or uneven cooling.

Temp sensors on these, if the ECU doesn't see it, it will automatically turn the fans on. Water in engines is a no-no. Drain it, put in the correct coolant.

It wasn't low in coolant. It hasn't been leaking, just started. All the problems started after he flushed the system (several days later). He put stop leak in it today and will get a new radiator.
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He has a lot of brown gunk in his radiator and coolant resivore. Where is the temp sensor and can I test it?

Brown gunk will cause over heating. It blocks the coolant flow in the radiator. Now is the time for a good flushing using chemicals.

You need an ohm meter to test the coolant sensor, but you also need the test chart that goes to that sensor. Depending on what the temperature of the sensor is, will directly reflect what the ohm reading will be.

He has already flushed it but I think he needs to do it again. Is the temp sensor next to the thermostat?

Yes, but if you are still seeing crap in the radiator, it will not cool the coolant down. It needs to be CLEANED out first before looking at the temp sensor.

Your right. I told him he needs to do the super flush/ reverse flush when he replaces the radiator. Hopefully that gets all the gunk out of the engine.