Check engine code po125 - 97 GP 3.8 - heater blows cold as engine temp rises to redline, then kicks in HOT and engine temp drops to normal... REPEAT. Is this the code for coolant temp sensor? Where is it and how do you swap it out? What else could be going on here? I've had most of the cooling system replaced (rad, hoses, thermostat). Thanks for your help, Amy.

Low coolant, if there is an air pocket in the engine it will make it to the heater core. Once there , the air coming out is cold/cool. Once the coolant fills the heater core, it gets hot. If the coolant is fine hot or cold, then have the HVAC system scanned.

Thermostat may not be working properly, hot coolant has to circulate via the heater core in order to get heat. Normally operation temperation should be 180 to 200 degrees.

Your temp sensor has nothing to do with your heater. The temp sensor needs coolant in order to work properly. So first check the coolant level because if it is low your heater will act the way you describe. Once the coolant gets low enough the heater will blow cold. Another possibility is the radiator cap. After the engine has warmed up it expands when it cools down it contracts. As it is contracting if the cap is weak air can be drawn into the system and cause air pockets to form and will cause the same problem

Hi, check your coolant level. When the coolant level falls below a certain level the heater core is bypassed. It would also explain your truck overheating if the coolant level was low. If your coolant is that low, check to see where the coolant is going. External leaks are not too bad, but those engines (4.3 and 2.2) were prone to internal gasket leaks also. Check your oil, it shouldn't be milky white.

think it needs more coolant added when engine is cold

dear friend-there is a couple of things you can do to find the problem,when the engine is cold remove the radiator cap start the engine then look inside to see if the water is circulating,all so look for bubbles in the water,if there is bubbles,you might have a blown head gasket,if there is circulation the water pump is ok,then get the engine up to working temp,then feel both the top and bottom radiator they should be the same temp,if not then the termostat is at fault

Your thermostat may be stuck in a partially opened position. This would account for the low temp while driving.You can check it by removing it (it should be closed when at room temp, if it's not, it's defective) and dropping it into a pot of boiling water, it should open completely within a few seconds. Also make sure your coolant level is correct, and when idling, check to make sure the engine cooling fan is kicking on when the engine temp rises.

You may have an air pocket in your system which some times happens.  One way to get rid of it is to take the upper hose off on the engine side,  fill your system with water until it runs out from your engine block, replace the hose, start your car turn you heater on and set to hi keep the radiator cap loose until the temp start to rise then close tight.  Once the air pocket had been remover you can drain some of the water from the radiator using the lower drain plug and add antifreeze.   

check coolant level ,must be low ,chck for coolant loss

Check to make sure your thermostat works right by monitoring the warming up of your engine. With a completely cold engine, start and run the engine for a few minutes, then put your hand on the top radiator hose to monitor if and when it gets warm as the thermostat opens more and more and note the gauge reading( if it works) the top radiator hose should be unbearably hot within 5-8 minutes. If the therm. opens right it should close right aswell, staying open a little while eventhough the radiator may have cooled may be what you are experiencing causing the rapid cooling and is nothing to worry about. The Hose should not get warm very soon after starting the engine. Regarding you gauge, lets take one step at a time, verify proper op. or the therm.