Does my heating, ventilation, and air conditioning system use relays or a control board to switch the blower fan.

Look at the air handler wiring schematic first because that diagram is the quickest definitive answer. Find the blower motor connections and trace the fan call input from the thermostat or control transformer; if you see isolated coil symbols or rectangular blocks labeled relay and contacts the fan is switched by relays. If the schematic shows a printed circuit board with multiple electronic components, connectors labeled with fan, blower, or G and you see triacs, transistors, or integrated circuits, then a control board is doing the switching. Physically inspect the air handler cabinet with power off and remove the access panel; relays are typically plug in or socket mounted modules that look like small rectangular boxes with visible spade terminals and a coil marking in a simple schematic on the housing. A control board is a flat board with silk screened traces and many components and terminals where harnesses plug in. While you are there, confirm line and load wiring sizes and that the blower motor leads are going to the device you identified. Use a multimeter on the low voltage terminals to verify whether the fan call energizes a coil or simply sends a logic voltage to the board, and if you measure a switching element on the high voltage side only after the coil is energized you have relays. If the high voltage is switched directly on the board without an intermediate coil visible in the low voltage circuit then the board is switching the motor. Keep safety first by killing power and verifying zero voltage before touching terminals.

Common mistakes are assuming that a control board cannot fail because it is electronic, mistaking a relay block mounted on the control board as a separate relay, and misreading schematics that use unfamiliar symbols. Thinking electronics are infallible leads you to chase wiring or motor faults when the board has burnt traces or failed components; inspect the board for scorching and measure outputs under load to confirm. A relay block attached to the board can look like the whole system is relay driven when the board actually governs the coil signals; follow the low voltage control path to see whether coils are being driven by the thermostat or by the board logic. Finally confusing connector pin labels with terminal functions causes miswiring during troubleshooting; trace each wire back to its source and compare to the schematic rather than relying on color alone.

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