2003 trailblazer - heat not working, blows cold air. After the car is driven the air is still cool, but the passenger side still blows really cold. System never gets hot. Back vents are blowing too.

the thermostat is either stuck closed (if slow to rise to the H) or open (if it never gets to the H)~ get a new thermostat...change the coolant with fresh~ https://www.google.com/#q=2003+Chevrolet+TrailBlazer+LS+thermostat&tb m=shop

it's overheating?...or no~?

suspect the coolant has NEVER been changed seaonally, and has turned to muck and boulders~ surprised it still acts like coolant and hasn't buggered your heat-exchanger, radiator and waterpump by now~

Thanks judge_roy! I'm not overheating at all. The temp rises to normal operating temperature without a problem. But after you mentioned the coolant, I looked and it doesn't look good. Funny I asked to have it changed with service back in September and they (SEARS) said it was fine. Also I couldn't open the radiator valve. It was so tight and wouldn't budge. I will have the fluid flushed for sure now. I had the thermostat changed about 2 or 3 years ago now. So, I remember my engine would never heat up to normal operating temperature back then. But the heat inside the car worked fine back then.

could it be that your heater core is plugged?...you might pull off the lower radiator hose (forget the stopcock) to drain and flush the heatercore (gardenhose will tell you/blast the corrosion out of there)~ try to save the coolant into five gallon buckets (gnarly as it is) for recycling at the auto parts store~ I know OLD coolant...have been running with the same coolant in my truck for at least 8 years...just haven't got around to changing that...just one of those things....of course my driving around here barely lets me reach operating temperature~....but take it in for a "coolant flush" and heater core flush...if they can do the job without messin' it up~

I think you may be right about the heater core. I bought radiator flush and tried that. Everything seemed to improve. But then, it doesn't blow hot air all the time. Now my engine temp went all the way up. When I checked almost all of my fluid was gone. I must have a leak somewhere and I'm sure flushing the radiator caused it. Wherever it was leaking from is now unclogged. It's a slow leak so it may take a while to find. Might have to take it to someone now.

I have had this problem as well I have had the water pump, thermostat,heatercord,radiator cap,fan clutch already replaced and it continues to heat up and the reserve fills up. Can anyone give me any other options i have done it all!