A stuck or welded relay on the control board can force the blower to run continuously by keeping the fan circuit electrically closed even when there is no demand.

A relay is an electromechanical switch that opens and closes a high current circuit using a low current control signal and when the relay contact welds shut or the coil sticks the blower motor sees uninterrupted power and will not stop. Start troubleshooting by turning off power at the breaker and visually inspecting the control board relay for burn marks, pitted or fused contacts, discoloration, or melted plastic around the pins, because those signs indicate thermal or electrical failure. Next confirm there is no active fan call by testing the thermostat and control signals with a multimeter on the control board terminals; the blower should only receive voltage when the control signal is present. If the relay looks damaged or the control signal is absent but the blower still has line voltage, remove the relay or board and bench test or replace it, since a welded contact cannot be safely repaired in place. When replacing, match the relay coil voltage and contact rating exactly and use proper anti-static handling and secure mounting to prevent vibration-related failure. Finally, after replacement, cycle the system multiple times and monitor for abnormal heat or noise and recheck control voltages to ensure the issue is resolved and not a wiring short keeping power on.

Common mistakes include assuming the thermostat or fan limit switch is at fault and replacing them before checking the relay, which wastes time and money, so always verify control signals at the board first. Another error is failing to cut power before inspection and testing which risks personal injury or further electronics damage, so always disconnect system power and confirm dead circuits with a meter. A third frequent oversight is swapping in an incorrect replacement relay with the wrong coil voltage or lower contact amperage which can fail quickly, so document the original relay ratings and wiring before ordering and install the exact match.

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