Blower Relay Troubleshooting — Indoor Fan Won't Run
The blower relay (also called the fan relay or IFC — indoor fan contactor) is an electrically operated switch that connects line voltage (120V or 240V) to the indoor blower motor when the thermostat calls for G (fan on) or when heating or cooling is activated. A failed relay leaves the blower motor dead — no airflow, poor heating or cooling performance, and potential equipment damage from the system running without air circulation. On most Carrier, Trane, Lennox, Rheem, and Goodman air handlers and furnaces with a separate blower relay, diagnosis takes less than 15 minutes.
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Common Symptoms
- Indoor blower fan not running despite thermostat calling for fan
- Outdoor unit running but no airflow from vents
- Fan only runs when set to manual ON, not AUTO
- Clicking sound from air handler but blower doesn't start
- Blower runs continuously and cannot be turned off
- Fan runs in one mode (heat) but not another (cool)
Most Likely Causes
- 1
Failed Blower Relay Contacts (Most Common)
The relay contacts pass line voltage to the blower motor. Contacts can fail open (no motor power) or welded closed (motor runs continuously). Test the relay contacts with a multimeter: with the relay energized, measure voltage across the load terminals — should read line voltage (120V or 240V). No voltage = contacts not closing. Motor runs with relay de-energized = contacts are welded.
- 2
Failed Relay Coil
The 24V coil that magnetically closes the relay contacts can fail open-circuit. With power off, disconnect the coil wires and measure resistance — a good coil reads 50–100 ohms. OL = open coil, relay must be replaced. With power on and 24V applied to the coil, the relay should click audibly when energized.
- 3
Missing 24V Signal to Relay Coil
The relay coil receives 24V from the control board when the G terminal is energized (fan call) or when heating/cooling is active. If the control board is not sending 24V to the relay coil, the relay never energizes. Test 24V between the relay coil terminals with the system calling for fan — no voltage means the issue is the control board or thermostat, not the relay.
- 4
Blower Motor Capacitor Failure
A failed blower motor start/run capacitor can prevent the blower from starting even when line voltage is applied through a good relay. If the relay closes (clicks, line voltage passes through) but the blower still doesn't start, test the motor capacitor MFD and check the motor winding resistance.
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Quick DIY Checks
The blower relay load terminals carry 120V or 240V line voltage. Always confirm power is off before touching any load-side wiring. Use a multimeter to verify voltage before reaching into the electrical compartment.
A relay with welded contacts causes the blower to run continuously and cannot be stopped by the thermostat or control board. If the blower runs non-stop and cannot be turned off, shut off the air handler at the disconnect or breaker — do not leave it unattended.
- 1Set the thermostat to FAN ON (manual fan mode). Listen at the air handler for a relay click within a few seconds. A click with no blower movement means the relay is closing but the motor or capacitor has failed. No click means the relay is not being energized — check 24V at the relay coil.
- 2With power off, locate the blower relay on the control board or as a standalone component in the air handler electrical box. Disconnect the coil wires (low-voltage, typically 24V). Set your multimeter to resistance mode and measure across the coil terminals. Good coil: 50–100 ohms. OL = failed coil, replace the relay.
- 3With the system calling for fan, set your multimeter to 24V AC. Test between the two coil terminals on the relay. Normal: 24V. Zero volts = the control board or thermostat is not sending the G signal. Check thermostat wiring: confirm G wire is connected at both the thermostat and the air handler G terminal.
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Try Pro — $7.99/mo- 4Test the relay contacts (load side) with power on and the relay energized: with the system calling for fan (relay should be closed), set your multimeter to AC voltage (120V or 240V as appropriate). Test across the load terminals of the relay. Should read line voltage when relay is closed. Zero volts = contacts are not closing, relay failed. Line voltage present but blower still not running = motor or motor capacitor issue.
- 5If the relay tests good and line voltage passes through to the motor, test the blower motor: disconnect the motor and test each winding resistance. Also test the motor run capacitor MFD with a multimeter in capacitance mode. A failed capacitor prevents the motor from starting. Replace the capacitor first — it's the cheapest repair.
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Repair vs Replace
A failed blower relay is a $15–$40 part and a 20-minute repair. A blower motor capacitor is $10–$25. Always repair before considering replacement — the air handler itself rarely fails due to relay or capacitor issues alone.
Est. Repair Cost
$15–$40 (replacement blower relay); $10–$25 (blower motor capacitor)
Est. Replacement Cost
$800–$2,000 for full air handler replacement
Recommended Tools & Parts
- Buy on Amazon →
Blower/Fan Relay (24V coil, 120/240V contacts, SPST or DPDT)
Universal replacement blower relay for HVAC air handlers. 24V AC coil, load contacts rated 20–30A at 120V or 240V. Match contact configuration (SPST or DPDT) and coil voltage to original.
$15–$35
- Buy on Amazon →
Blower Motor Run Capacitor (match MFD and voltage)
Round run capacitor for indoor blower motors. Match the MFD rating and voltage (typically 370V or 440V) to the motor nameplate. Weak or failed capacitor prevents motor starting.
$10–$25
Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.
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Frequently Asked Questions
- How do I know if the blower relay or the blower motor is bad?
- Listen for a click when the thermostat calls for fan — a click means the relay is energizing. Then test line voltage at the relay load terminals: if line voltage passes through but the motor doesn't start, the problem is the motor or its capacitor. If there's no click and no voltage passes through, the relay coil or the 24V signal to it has failed.
- Why does my blower fan run constantly and won't shut off?
- Continuous blower operation that can't be stopped has two common causes: (1) the relay contacts have welded closed — the relay is always passing line voltage to the motor regardless of the control board signal. Replace the relay. (2) The thermostat is set to FAN ON instead of AUTO — check the thermostat fan setting first. If it's on AUTO and the fan still runs continuously, test whether 24V is continuously present at the relay coil — if yes, the control board is sending a continuous fan signal (possible board fault or jumped terminal).
- Is the blower relay the same as the contactor?
- No. The contactor is in the outdoor unit and switches 240V to the compressor and condenser fan motor. The blower relay is in the indoor air handler or furnace and switches line voltage (120V or 240V) to the indoor blower motor. Both are electrically operated switches, but they are separate components in separate locations with different voltage and amperage ratings.