Indoor Blower Will Not Run

Tracing a dead blower across PSC and ECM equipment without guessing at parts.

Troubleshooting

Apprentice

#diagnostics#electrical#airflow#blower

Safety

De-energize and lock out before servicing. Discharge capacitors before handling. Do not permanently defeat the door switch.

Symptom

No air from the registers on a fan call, a heat call, or a cooling call.

Symptom

Or the blower runs in one mode but not another.

Possible causes

Door switch held open by a panel that is not fully seated.
No line voltage to the air handler or furnace.
Open control circuit, blown low voltage fuse or failed transformer.
Failed fan relay, blower relay on the board, or an open sequencer contact on electric heat equipment.
Failed run capacitor on a PSC blower.
Failed ECM control module or missing control signal.
Seized bearings or a blower wheel jammed by debris.
Open thermal overload in the motor after an overheat event.

What to check, in order

Door switch and panel fit

A very common no blower call. Hold the switch closed only as a momentary test, never defeat it permanently

Line voltage at the equipment and 24 volts at the board

Establishes which side of the system has lost power

Low voltage fuse on the board

A blown fuse points to a short in the thermostat or accessory wiring

Control signal at the blower terminal during the call

Signal present with no motor response moves the fault to the motor or module

Run capacitor on a PSC motor after discharging

Out of tolerance prevents the motor from starting and often produces a hum

Whether the wheel turns freely with power off and locked out

Drag or debris indicates bearings or a mechanical obstruction

Motor winding resistance and amp draw on a start attempt

Open windings or locked rotor current confirm motor failure

ECM module line voltage, polarity and commanded signal

Separates a failed module from failed windings

The door switch is a safety device

It exists so the blower compartment cannot operate with the panel off. Use a temporary hold only while attended and reinstall the panel before leaving.

Next step

If the motor overheated, find the reason. A clogged filter, a dirty blower wheel or excessive static pressure all overwork a blower. After the repair, measure total external static pressure and either temperature rise or temperature split to confirm the airflow is where it should be.