I have been trying to troubleshoot this for some time. I have a 2003 suburban with 5.3L engine. The engine never comes up to operating temp (regardless of how long I drive it). I started with swapping out the t-stat and just today the ECTS. Neither seem to have done the trick. The air coming out is warmish but on a freezing morning I doubt it will do much. I have seen it come up to temp once and as soon as I turned the heat on it blew HOT for about 3 seconds then the temp dropped off again. I did notice that the fan is running constantly even at a cold start. Any other spots to check? A relay? another sensor? Hoping it isn't the heater coil (which I am doubting) or the water pump as I get plenty of flow back to the reservoir when at idle.

 

Drive about 10 minutes, then feel the heater inlet and outlet pipes at the engine firewall. If hot, the issue is inside the dash, like a bad actuator. If not hot, issue is with the engine, like bad thermostat or air in the cooling system.

 

If the fan is constantly on you need to check the sensor on that circuit. You could try unplugging the fan and see if it comes up to temp, but DO NOT drive it that way