Drivers And Passengers Side Vent Temperatures Vary By More Than 20 F (11 C) (Without A/C)

Driver's and passenger's side vent temperatures vary by more than 20 °F (11 °C)

Probable causes: The recirculation control door or the air mix door is malfunctioning

Do the following troubleshooting:

Recirculation control motor circuit troubleshooting Recirculation Control Motor Circuit Troubleshooting (Without A/C)

Air mix control motor test Air Mix Control Motor Test

Also check for

Control panel DTCs How To Troubleshoot The Heating System (Without A/C)

Also check for

Poor or loose connections at the terminals