I have a central A/C (about 15 years old) and forced gas furnace (less than 1 year old). It started last week that the A/C compressor won't turned off even when the thermostat setting was reached (and the blower turned off). I also noticed the refrigerant pipe from the compressor into the air duct (and the coils inside there) had ice on it so I assume the compressor worked all right.

I checked the fuses for the interruptible power and those seem to be OK - I can pull the fuses out to shut off the compressor or put them back in to start the compressor.

So it seems the control between the blower and the A/C compressor is malfunctioning. How do I find out what to do next?

Thanks!

Most likely cause is the contactor outside in the AC unit. The contacts have failed to release. There is 240 V at high amperage available there as well as 24 VAC. Power comes from two places: 24 VAC from the downstairs unit and 240 from the box on the wall outside.

Bee careful. With power off see if the contacts are fused together.

This is the best guess, not all possibilities.

Thanks a lot. I will pull the fuses out before I check. I will update you. Thanks again.

I opened the cover and checked the contactor - there is a black burn mark and it was stuck closed. Thanks so much for helping!