Error Code 40 on the Thermostat

Article ID: 32
Last updated: 16 Dec, 2022

Honeywell Thermostats: VisionPro IAQ

Re: Thermostat Shows Error 40

Some heat pumps have a diagnostic board that will flash a light or display "Error 40" at the thermostat when a fault condition occurs.

  • Error Code 40 is displayed on a Honeywell VisionPro IAQ thermostat when it receives a 24V signal on the "L" terminal.
  • The actual Honeywell description of a Code 40 is "Compressor not operating when it is supposed to be".
  • This is typically associated with a fault from the outdoor unit.

Fault Codes: 

  • Any fault condition that activates the "L" terminal will cause an Error 40.
  • In order to see the actual fault code you must visually check the outdoor unit control.
  • Most fauts can be Reset by cycling power to the air handler.
  • Some codes can be Reset by turning the thermostat System switch to "Off" for 30 seconds and back on.

What the Customer Should Do:

  • Check/replace air filter.
  • Check breakers.
  • Reset the lockout by one of the reset methods mentioned above.
  • Make sure the indoor blower is operating.
  • Check the outdoor unit for any abnormal conditions.
  • If Error Code 40 returns - have a technician check the equipment.
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Article ID: 32
Last updated: 16 Dec, 2022
Revision: 8
Views: 1080
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