Why is there no hot air coming from the heater in a Mazda RX-7 FB?

On the FB RX-7, a lack of heat usually comes down to a mechanical failure with the heater control valve or a fundamental cooling system issue like a stuck thermostat.

Here is how to diagnose it, in order of what is most likely to fail on this platform.

Difficulty: Easy to Moderate

1. Check the Heater Control Valve and Cable

The temperature slide on your dash operates a mechanical valve via a cable, which physically opens to allow hot coolant from the engine into the heater core ². Over time, these valves seize up, or the control cable bends instead of pivoting the valve arm.

To test this, look under the driver side of the dash at the side of the center console. You may need to remove a plastic cover to get a clear view of the valve ⁴. Move the temperature slide on the dash back and forth from cold to hot. Watch the rod on the valve—if it isn't moving, your valve is either frozen shut or the cable has disconnected/failed. If the valve is seized, you can sometimes free it up by hand to get heat temporarily, but it will likely need to be replaced.

2. Verify Thermostat Operation

If your temperature gauge is reading unusually low, your engine isn't reaching operating temperature. A stuck-open or missing thermostat will prevent the coolant from getting hot enough to transfer heat into the cabin. If this is the case, replacing it with a standard 180-degree thermostat is the fix ³.

3. Check Coolant Levels

If the system is low on coolant, it won't circulate high enough to flow through the heater core ³. With the engine completely cold, open the radiator cap and ensure the system is fully topped off. If it was low, you may also have an air pocket trapped in the heater core, and the system will need to be bled.