My girlfriend's heater doesn't work, and hasn't since she bought it. She graduated, is paying back loans, and saving money to get something better.

She says the heater doesn't work, and it may not, but adjusting the thermostat inside the jeep from cold to warm doesn't seem to do anything - it's always full cold. I'm thinking it may have more to do with that and less to do with the actual heater.

How would I go about testing this hypothesis?

Sometimes, people will bypass the heater if it develops a leak. Look to see if the heater has been bypassed. At the firewall of the car, there should be two hoses going to it that go to the engine.

1) Check the fluid level. Sometimes folks let the coolant level drop so low that the heater fails to work, or works intermittently.

2) Find the heater hoses. There should be two hoses, one to and the other from the heater core, which could be in the engine compartment or under the dash.

3) As Mr. dubi said, see if the hoses are connected. If they are blocked off, a bad heater core is likely the reason why.

4) If connected, run the engine with the heater on long enough to come to operating temp and open the thermostat. Carefully feel of the hoses. The one going TO the heater core should be HOT. The FROM hose should still be warm. If one or both are cold, you have a blockage.

5) If you have cold hoses, look for a valve on the hoses and check to see it it operates.

6)If hot water is going in and warm coming out, the next thing to check would be the air deflector doors in the heater.

[This message has been edited by CanyonAg77 (edited 1/26/2011 9:56p).]

Do the ZJs have blend doors?