To test if a Honeywell thermostat is failing, start by checking the power supply and replacing any dead batteries or resetting the breaker. If the display remains blank or the HVAC system does not respond despite correct settings, remove the faceplate and use a digital multimeter set to AC voltage to check for approximately 24 volts between the R and C terminals. Next, test continuity or voltage output across the R and W (heating) or R and Y (cooling) terminals while calling for conditioned air. Verification: Confirm that no voltage or signal output is detected at the terminals despite proper incoming power, which indicates an internal device failure.
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