As I understand it, it's a common occurrance to get a 075 error on these cars. I replaced my duo-valve at a considerable expense to try to get rid of the code, and got the code back within a few days of the installation. I've been told on more than one occasion "don't worry about it as long as your heat works correctly."

If you've got unbalanced heat, the duo-valve is where I'd start.

To confirm, you can also check under the hood while the car is at operating temp - there are two hoses coming from the heater core (engine coolant flows into the core in a single pipe and exits to the duo-valve through two pipes). Having an infrared thermometer helps, but you can test it with your hand if you're careful. The pipes run along the bulkhead to the right of the throttle actuator. They're about 1/2" (26cm) in diameter. See if one is noticeably different in temp than the other. If so, either the duo-valve is malfunctioning and not allowing the coolant to flow out of that side of the heater core, or you've got a blockage.

Pulling sensor values will also help. Dials to white - CC on. Press and hold Rest for 5+ seconds. Cycle through the codes with the left Auto button. Register 3 is the left heater core temp, register 4 is the right heater core temp. If the temps say the same, then likely the sensor for the right core (if the right side is the one that's cooler in the car) is defective and the CC thinks the temp is right.