I have a 2003 Impreza 2.5 TS. My heater hasn't been working consistantly under the following circumstances:

when my car has been parked outside, in the cold for awhile, on a hill (even a moderate one).

I'm finding under those conditions, it will take about 10 miles of driving before my heater kicks in. Even then, after it has begun to heat up, when it idles (at a stoplight for example), it will pretty immediately begin to blow cold air again.

I haven't had this problem at all after it's been parked on level ground.

any thoughts?

Thanks!
-Laura

 

First thing to check is the coolant level. If it's low it can cause this. Don't just look in the reservoir, take the cap off and look in the radiator as well (do this only when the engine is cold). If it's low add coolant in a 50/50 mix so the radiator is full, then top off the reservoir as needed. Recheck after driving and allowing the engine to cool down, you might have to repeat this a couple of times.
If you do find the level to be low, check for any visible leaks, tighten the hose clamps etc. It's possible to have a slow leak, such as from the water pump or a head gasket, where coolant seeps out over time but you don't see any on the ground.

 

thanks mulder for the tip! i had looked in the reservoir, but will check in the radiator too.