Heater only works sometimes.

Engine will be up to temperature but system blows out cold air.

Coolant level is good.

What else to check?

It has the "auto" option where you set the inside thermostat or manul adjust, but neither one works.

Then all of a sudden it works fine for a while, but sometimes you may drive it for an hour before the heat decides to work.
Any thoughts?

check for any leaks and check your thermostat.

thanks
pat

3 possible things to ck
1. Obstructed cowl air intake
2. Obstructed heater system outlets
3. Blend-air door malfunstion.

The procedure totest the blend-air door operation is diffrent for ATC and nonATC systems. Which do you have? (Do you have 3 big knobs and two buttons (AC and rear defrost), or ATC with 10 buttons and two knobs with a digital display?)
Let me know.