Had this problem a year ago but it resolved itself then but don’;t know how. Heater only blows cold air. What does the temperature knob control?
Have you checked the coolant level?
From what I see there’s a heater control valve and 3 vent door actuators: mode/recirc/& mix. Most likely a door actuator isn’t working, or the control valve is malfunctioning. If those test ok, next you’d be looking at the ambient temp sensor, cabin temp sensor, dash control ass’y.
Before stafting in on all that:
- Is the engine reaching normal operating temperature?
- What’s the temperature of the two heater hoses that go through the firewall?
Thank you for your assistance. Engine is getting to operating temperature. Heater hoses along firewall in engine compartment both warm/hot; Radiator overflow has antifreeze at proper level. Do you what/hoiw the temperature control knob (rhs) operates? Dopes it control a valve that lets more or less goolant water through the heater box?
How can I test the items you identified?
Yes it appears normal level in overflow tank
The heat control (cold-to-warm) on my 1999 Civic operates a valve near the firewall, controlling coolant flow from the block to the heater core. Yours may be similar. With the ignition on, you can have someone operate the cold-warm knob while you watch the action at the valve near the firewall.
It’s one reason I bought the car. I did not like the radiant heat that is always present in cars that have the heater core always getting full flow, and use doors and the fan to reduce or increase the air flow over the hot heater core and into the cabin.
Thank you for your assistance. I will do as you suggest and advise.
The HVAC control unit operates a servo motor that is connected to a blend air door. The blend air door servo motor may be sticking.
Sorry, I couldn’t find a description of how exactly it works. I expect it is some combination of the flow rate of hot coolant going through the heater core and position of the vent doors. @shanonia 's comments above about trying to observe the coolant flow valve as the knob is turned are a good idea. That would work on both my Corolla and my truck, the valves are clearly visible and what they do is obvious. It’s possible that on your car the valve is an off/on type rather than a proportional flow valve. Or there may be no coolant flow control valve at all, and it is all done with the vent doors. Based on what you say above, my guess this particular problem is a vent door control rather than a coolant control valve problem.
Thank you, can you describe where the servo is located? Please