Driver's And Passenger's 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 cause: The recirculation control door or the air mix door is malfunctioning

Check the operation of the recirculation control cable and linkage Adjustments. Adjust or repair as needed

Check the operation of the air mix door cable and linkage Air Mix Control Cable Removal/Installation. Repair as needed

Also check for

Faulty heater controls Overhaul