2001 blazer DTC codeP0128

[rmxsikorski]

hello, i have a 2001 blazer 4.3 .had no heat,changed thermostat, flushed heater core twice sometimes heat. check engine light came on ,was oxy sensor, replaced, passed nys inspection still no heat. a couple days later check engine light came on again, dtc code p0128, low coolant temp. i checked thermostat on the stove with boiling water was ok. bleed and topped off coolant, drove around for 1/2 hr . temp was reading around 160*? low or no heat any suggestions???? *****HELP PLEASE******

[DeltaP]

Check both heater core hoses. Is temp the same on each hose? If not heater core clogged. Back flush. Make sure heater is set wide open when refilling and bleeding system. If I was removing the thermostat to test it I'd be replacing it. What was the rated opening temp on thermostat? Water boils at 212*F. It's gotta be cold as balls in Buffalo now! I went to school there at SUNY.

P0128 can be caused by:

Low engine coolant level
Leaking or stuck open thermostat
Faulty cooling fan,(running too much). Or it's control circuit.
Faulty coolant temperature,(ECT) sensor. Or it's circuit wiring.
Faulty intake air temperature,(IAT) sensor. Or it's circuit wiring.

[rmxsikorski]

thanks deltap, it is a new thermostat 195 degree, thought i would check it to be sure.I flushed heater core 2x guess i should do it again. fan seemed to be running too much. after i bleed system, seemed to be running less. was 8 degrees at 8 this morning, warming up to 38 later.ill let you know situation if any thing with car changes, thanks again.

[Tech II]

Most important, anytime you open up the system, to thoroughly bleed the system of all air.....

You say you back flushed the core already, but did you test it? I always had extra hoses around....attach one to the outlet and blow through......should be absolutely no resistance whatsoever.....then blow through the inlet and should be the same....if not, there is some crud still in there......

Then I attach the lower hose and put vice grips on it.....hold the other hose just above the other heater core nipple....then I fill my Lisle funnel(this has adapters, so you remove the radiator cap and then attach it)..... when coolant comes out the top hose, I clamp it with vice grips, close to the end......then I remove the vice grips from the lower hose...when coolant come out the other heater core nipple, then I remove the vice grips and attach the other hose and all air is vented....then attach clamps to the heater core hoses.....then start the vehicle any remaining air will bubble out the funnel.....the coolant level in funnel will rise as temp rises, so do not have it filled too much when you start the car,,,,as car starts, it should be just filled to the bottom....the coolant will then rise until the thermostat opens, then the level will drop...it may drop so much that the funnel empties,,,,then fill with coolant until the radiator is full....with vehicle still running, quickly remove the funnel and attach the radiator cap......make sure there is coolant in the reservoir(later as coolant cools, it will suck coolant out of the reservoir).......when engine is cold, make sure the level in the reservoir is at the "cold" level.......

See if you have heat now out of the heater core, when you put the radiator cap on....if no, feel both hoses...if both hoses are hot, then you have a temp door problem....

If you go to the Lisle website, they have a little video showing how to use it....

[rmxsikorski]

thanks for the info techII,Ill let you know when and if problem is resolvep