Can a faulty terminal block cause the blower fan to stay on? Yes, and it is one of the hidden faults that creates tricky intermittent issues.

A terminal block can create a short between the thermostat fan wire and the system fan relay when corrosion, loose screws, or broken insulation allows the fan circuit to bypass normal controls. Start by turning power off at the furnace switch and the breaker, then remove the access panel and visually inspect the terminal block for greenish corrosion, melted plastic, or frayed conductors. If you see any of those signs, gently clean corrosion with a small brush and a contact cleaner rated for electronics and retighten screws to manufacturer torque if available, otherwise snug but not overtighten to avoid damaging the plastic. To test isolation without a thermostat, remove the fan conductor from the terminal block and temporarily jump the appropriate fan terminal to the power terminal with a short insulated wire to verify the blower runs only when commanded. Use a multimeter on the system voltage range to check for voltage between the fan terminal and common while the blower is running; a persistent voltage when the thermostat is not calling indicates a shorted connection or internal relay fault. If cleaning and tightening do not stop the blower from running, replace the terminal block or the furnace relay board using the exact replacement part, and keep wiring labeled and routed to avoid future chafing.

Common mistakes include assuming the thermostat is at fault and replacing it without inspecting the control board, which wastes time and money; isolate the fan conductor at the terminal block first to prove where the short lives. Another issue is overtightening terminal screws and cracking the plastic, which creates intermittent contact and shorts; use proper torque and replace damaged terminal blocks rather than reusing broken plastic. A third concern is failing to de-energize the system before testing or moving wires, which risks damage and injury; always cut power and verify with a noncontact voltage tester before touching wiring.

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