A/C Off Button just quit working!!!
 

The car is a 2002 C240 (W203). The A/C Off/Rest button on my hvac control just quit working! The small red light on the switch is on and will not go off, so the best I can get is the vent - or heat - depending on outside temperature. The is the second set of hvac controls in this car. Any ideas?

Is your car equipped with automatic climate control? If so, the pushbutton unit can give you diagnostic information in the form of trouble codes and current-sensor values. The procedure for your car is on here somewhere so I would try a search.

If you don't find anything, I can look up the procedure in the climate control manual later.

The system has sensed a low refrigerant level or the sensor is faulty. Have pressures read and/or fault codes. The acc head is not the problem.

My 2002 S-500 drove me crazy with that. Yours might operate the same as mine, here is what I learned...

If the CCU senses something wrong, it will lock out the compressor and turn on the A/C OFf light. In my case, the light would not reset until the shop used the diagnostic computer to clear the codes. It did not display a code, but clearing them reset the CCU. They said my firmware of CCU has an error condition that doesn't display properly. The newer version CCU presents the code properly.

In my case, it was an intermittent Evaporator Temperature Sensor. I think low R134 will show the light as well.

Good luck! These things have become exponentially more complicated than they were just a few years ago!