Originally Posted by Whiskeydan

I'd suspect a stuck duovalve. You might try this...

With both sides set to cold, cycle the valve several times by disconnecting and connecting the valve rapidly. I had a stuck valve free up by doing this and has been working fine ever since.

If that doesn't fix it measure the valves coil resistance. Should be 30-50 ohms from center pin to each side. The valve's solenoid coil can open which requires a new valve.

If coils are good then measure voltage at the connector to see if there's 13 volts on each side. If so, then the valve is stuck and you may be able to dismantle and clean it.

If you see 13V only on one side then there may be climate control, wiring problems.