Returned to complete furnace repair. Continued diagnostics to confirm diagnostic. Found we can manipulate wall control to lock out furnace at 40 degrees. Cannot change temperature adjustment for backup heat past 1 degree. Lockout at 40 allows us to change set temperature as warm as needed without calling for gas heat as long as ambient temp is above 40. Between 35-40 customer understands +2 degree difference will turn on backup heat.