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.

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1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350

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!

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)