I have a car question. I have an 03 Chevy Impala. The car is running fine, starts up, idles okay, isn't running hot. How come the heat in my car only works when the AC is on? If I turn the dial to just heat (on any of the settings) its cold, but as soon as I hit the button for AC the heat comes out full blast and is HOT. What would cause it to do that??

sounds like you have computer problem's ???

But wouldn't that make the car not run right?

It's probably a burned up or broken heater blower motor.There is a small blower that only pushes air through the heater core which is what gives you your heat.Your A/C unit also has a blower motor which is probably routed through the same air ducts as the heater core.Even with your A/C on if you have the switch to heat it will probably blow somewhat hot air since the air is likely close or through the heater core.Check these things....

1.)Turn switch to highest level with out A/C on and see if air comes out the vents.If air comes out the vents your heater blower motor is fine.

2.)If car is warmed up to normal operating tempeture and the blower is on and there is no heat then it is either a bad heater core or a problem with the fresh air vent gate being stuck either closed or open.If the fresh air vent gate is stuck it could be blowing your hot air back to your engine compartment.

I think your problem is the gate for the A/C is stuck only for the A/C.It should close when the A/C is off to allow fresh air to bypass the A/C and let air pass through the heater core.

Air does come out the vents. The other thing I noticed is that once I get up to around 45-50 mph the air blows hot on both heat and AC.

it could also be the heater core modulator might need to be replaced as well. You shuold really have a mechanic check it out for you.

Yea, I agree. Need to find a new mechanic, mine seems to think I am made of money.

do a vacuum test.....about 50 cents worth of rubber hose.....is what I would replace first.

Go by bike or use collective transport. That is much better for the environment and for your health. It has been proven that yuor risks of heart disease are drastically lowered.

Ck local Better Business Bureau for small shops, with a good rep.
Otherwise, dealers are expensive; but have access to all the top repair info.
also ask friends & co-workers where they take their cars.

The heater/ac control panel is probably no good

Thanks. I think my car is possessed or something. Today the freakin heater worked perfectly fine, on Wednesday it didn't get hot all on heat or with the AC button pressed.

Thanks for the tip, I'll check it out. I was actually taking my car to a locally owned "family" type garage, but even if I take it in for an oil change or a tire problem they ALWAYS come up with something else to fix.

Not sure but I went to autozone where I lived. They hooked my car up to some device which gave me a printout as to what was wrong with my car. They did this for FREE.I gave this to my mechanic who fixed the car.

Wow, thanks. I didn't even know I could do that. I'll definitely check out that option.

Sounds like a vacuum hose prob to me... I'd do a vacuum test like that other poster recommended. It could be an easy fix.

I'd need my mechanic for that...have no clue how to do something like that.

It can be tricky but what's probably happened is that a vacuum line has either come loose, or one has failed due to a crack or cut. The little black rubber vacuum lines are about a half inch thick and run from your vacuum booster, at or near the brake master cylinder and run off towards other pressure servos that control your heat and ac circulation, a;long with various other ingine components.
It could be as easy as looking for a loose hose and reconnecting it to a likely looking tube in the general area as the end that's become disconnected or loose.
Your vacuum lines are located under the hood and under/in the dash.

Sounds like I should hire you to be my new mechanic.

yeah, that'll fix the problem for sure.