2004 Trailblazer sputters and often stalls if fan, defrost, heat or air are turned on. Service engine light is on and coding for enine running too cool. Are these issues related and/or what could be causing the stall.

Do you have heat? Does the fan turn on? are you getting air from the defroster vents close to the windshield? If you have heat make sure the defrost control is working. I'd open the outside air vent and turn the blower on high speed. If there is no heat, the heater control valve is not working. Double check the radiator fluid tank, make sure you have enough fluid (antifreeze) Low fluid levels can reduce the hot water flow to the heater coil inside the car. If there
is no air flow through the defroster vents you may have a problem with the fan control, or the fan motor. Good luck.

Have the codes checked. Those lights come on for a reason. And those numbers will help us, help you spend less money.

Any codes? Fuel filter and air, dirty throttle body.

You say the engine service light cam back on. Was it on before? Did you have the computer scanned to find out what the light was for? Changing the thermostat and coolant wouldn't have anything to do with the engine dying. What sensor did you replace. When you turn on the defroster, the air conditioner also comes on. Engine idle speed is supposed to increase when the a/c is on so the engine wont stall. If the computer is not increasing the idle speed,that may be the reason the engine is stalling. Need to get those trouble codes.

6 YEARS IS A LONG TIME FOR A BATTERY TO LAST. YOU ARE LUCKY. I WOULD GET A NEW INTERSTATE BATTERY & CHECK THE CHARGING SYSTEM, TO MAKE SURE IT IS OK. MOTOR RUNNING, THE VOLTAGE AT THE BATTERY SHOULD BE BETWEEN 13.90 & 14.80.

You need to get the fault codes read to determine where the problem is that set the SES light. Autozone will scan your car at no charge. At some point you will need to get a mechanic involved to further diagnose the codes.

When the check engine light is flashing, it indicates a serious condition. You need to have the car scanned for trouble codes and diagnosed, making guesses is going to waste money.

if this happens while the defrost of ac is turned on, it could be a seized ac compressor which will cause the serpentine belt to squeal and drag making the engine sometimes stall. if it happens with the ac off and the selector on say the floor, or dash vents, the issue could be a bad alternator and battery. this is confirmed when the blower motor engages and the lights dim and then the truck goes dead.

Defrost also work's a/c compr. could be a/c compr putting to much load on engine, make sure engine is running on all cylinder's, also have a/c compr check with a/c gauges.

try changing the air filter ,i had a problem ,at ilde in drive my trailblazer would stall, changed the air filter now it's not stalling and got my power back ,there is a secondary air pump that i have to change ,
not sure were it is ? still not running perfect but it's running better .and my service light is on too.
i got it put on scope ,it's the pump .
not sure if this helps ,but i'm start to run in to problems too,i have a 2004 trailblazer.
i like to do fix things myself ,i hope i can.it's the cost if you know what i mean.