3716 TC reported cab air con was warm and air-con fault. ~~~~~ 3716 TC reported cab air con was warm and air-con fault. Unable to reset. Staff (ADM) feedback no air-con fault found at the rear DT car.Car was cooling. CE:3716 Cab AC  No faults lit Suspect freon low TEmp 28.6  N Checked Cab AC - OK. Able to cut in/out. Found cab filter very dirty. C/O both cab filters & function tested-OK.