the manual ones usually just get worn over time, the "turn" knob has a copper leg that switches position inside the control to a corresponding copper leg over time they get **** built up and maybe even burnt some, this causes loss of contact and the control then may only operate sporadically. the plastic bits inside also become VERY brittle so if you open one up you have to be very careful.
the digital ones are more prone to failing electronically and the lights burn out. one thing they will do is work intermittent, it may work upon a key cycle for a few minutes or may just stop working mid-drive until you cycle the key again, this often create a headache because then you are chasing a ghost thinking the ac is low or pump not working etc...
then.... in either analog or digital on the gmt800's the blower motor resistor takes a crap and throws another troubleshooting step in the mix
