What happens
A loose or corroded connection restricts current without being completely open.
Typical symptoms
Thermostat resets, relays chatter or controls work unloaded but fail when a coil picks up.
Checks that distinguish this cause
A qualified technician compares voltage before and after the connection during the failing load condition. Inspect and repair with power isolated. An unloaded continuity test may miss the fault.
What can look similar
Excess transformer loading also lowers control voltage, but does not necessarily localize a drop to one splice.
Repair direction
Renew the damaged connection using approved connectors and the specified termination method.
Verify the repair
Confirm the localized drop is resolved and the load remains energized reliably.
Equipment-specific limits
This guide addresses conventional R, C, Y, G, O/B and Y2 circuits. Communicating buses require their own service procedure; never apply jumpers to unidentified terminals.
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.
- Fluke: HVAC and industrial electrical testing applicationsElectrical measurement principles, including control-circuit voltage references.
- Resideo: RTH6360 thermostat installation and setupExamples of heat-pump O/B and staging setup. Menu numbers vary by thermostat.
Editorial review: 2026-09-29. Verify the current manual for the installed model.