A homeowner in Apex called us about a heat pump that wouldn't switch out of cooling mode in 38-degree weather — blowing cold air with the thermostat set to 70. Another company had already diagnosed a failed reversing valve and quoted a $2,800 repair, or full system replacement at $11,000.
We came out with a $72 diagnostic. Gauges on the line set, multimeter on the defrost board, voltage check on the reversing valve solenoid. The reversing valve was functional, the solenoid was getting voltage, but the defrost board was sending stuck signals that kept the system reading "in defrost cycle" indefinitely.
Replaced the defrost board, recalibrated the outdoor sensor, and tested through three defrost cycles. The system swapped between heating and cooling modes properly. The $72 diagnostic credited toward the work. The reversing valve, original from a 2018 install, is still in service.