Error Code 40 on the Thermostat

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.