What happens
An open R or C path, missing transformer supply or depleted batteries prevents normal operation.
Typical symptoms
Blank display, reboots, lost settings or intermittent calls.
Checks that distinguish this cause
Identify the thermostat's actual power arrangement and compare power at its terminals with the equipment source. Inspect batteries only when the model uses them.
What can look similar
A working battery display does not prove equipment control power exists. Some power-stealing models have compatibility requirements.
Repair direction
Repair the supply path or replace batteries as specified before condemning the thermostat.
Verify the repair
Verify display stability while cooling and fan commands are issued.
Equipment-specific limits
Confirm the equipment type and supported stages. Anti-short-cycle delays, schedules and lockouts may be normal behavior.
Practice this diagnosis
This cause is a reference topic; it does not yet have a dedicated simulated call. Browse the practice board.
References and applicability
Original HVACSim explanations with manufacturer and instrument-maker references for further reading. These resources support the subject; their model-specific examples are not specifications for every system.
- Resideo: RTH6360 thermostat installation and setupExamples of heat-pump O/B and staging setup. Menu numbers vary by thermostat.
- Fluke: HVAC and industrial electrical testing applicationsElectrical measurement principles, including control-circuit voltage references.
Editorial review: 2026-09-29. Verify the current manual for the installed model.