The HVAC and control board is a McQuay Mark IV. I'm not sure if I would like to try and fix this myself yet. Just trying to figure out the issue before deciding where to go.

Here's the scenario:

  1. Switch thermostat to Cool setting.
  2. Set temperature to 5 degrees below current temperature.
  3. HVAC fan starts to kick in.
  4. 15-20 seconds later, AC compressor kicks in.
  5. 30-60s after compressor kicks in, the contactor sparks and makes a short 1-2 second popping/buzzing/humming sound, a red LED light flashes once, and the compressor stops.
  6. HVAC fan continues to run.
  7. If left running, the HVAC continues this nonsense and repeats from step 4.

Can anyone help diagnose?

Other things to consider:

A Nest thermostat was installed during winter. The heating works fine. Tested out the cooling and couldn't remember having any issues then. The installation involved plugging in a blue wire as common for power source, which was spliced from the bigger brown wire that runs to the thermostat. It was plugged into the C terminal. Also, the super came by once time when there was no heat coming through and fiddled with the pipes and drained some liquid out before the heat returned.

2 Answers 2


After checking with the building maintenance, the super said, because the building's main system is in "heating" mode and haven't switched back to "cooling" mode, the HVAC will automatically override and turn off the compressor. The building officially switched to "cooling" mode yesterday and my HVAC is operating as normal again. As it turns out, it is the building's main override.


Yeah, the contactor is damaged -- tell your super to replace it before it decides to burn up completely. The red LED is probably the control board sensing some sort of problem, by the way.


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