The heat quit working in my Accord yesterday. It had been working without any issues until then. I checked that the control valve in the engine compartment is moving, and that both actuators are working in the passenger compartment. The airflow does change, when I push the buttons on the control panel (defrost, upper, lower, both). The fan works fine. The thermostat is good. What else should I be checking? Thanks

 

Are you getting air flow to the proper location, but the air temperature is not hot?

 

Yes, that's right. Air comes out from all of the correct places, but it is cold air

 

Does your 99 have dual climate control where the driver and passenger side can have different temperature settings?

 

look if the heater valve is moving, it's located on the pipe that's coming from the firewall on the passenger side.

 

No, it doesn't have dual controls.

 

Yes, the valve in the engine compartment opens and closes when the temperature dial is turned while the ignition is on. The lever on the valve doesn't seem like it's loose at all, either.

 

With the car running at operating temperature what where is your temp gauges at. What is the temp on the hose going to the heater core? Only if you have verified your thermo is not stuck open then it's either you climate control, air in system, or blend door actuator on the passenger side.

Troubleshoot the actuator first. Remove it from its location then plug it back in and operate the heater with the car on. If it moves then it's working if not then replace it. If your heat was working last year and you had no leak or didnt do a coolant change then I wouldn't worry about air in the system. Now it points to the control unit.

 

maybe the heater core is clogged.

 

The thermostat is good. The engine temperature is good. The drivers-side actuator functions well, and the air is directed correctly. The passenger-side actuator is working properly, and the cable is opening and closing the valve in the engine compartment. The control unit is fine, or neither actuator would be working. Thanks.