Goodman furnace b5: Blower motor locked rotor
b5 means the motor tried to turn and could not. Something is physically stopping the blower, or the motor has seized.
Applies to: Goodman GMVC96, GMVM97, GCVC96, GCVM97 and Amana AMVC96, AMVM97 series gas furnaces with a variable-speed ECM blower motor and a two-character display.
What it means
Locked rotor is exactly what it sounds like: the motor applied power and the shaft did not turn. The electronics detect this immediately and stop, because a motor stalled under power heats very quickly and will destroy itself in a short time. Unlike b4, where the motor turned but exceeded a limit, b5 means no rotation at all. That narrows the causes usefully — either something is jamming the wheel, or the motor's bearings have seized. Both are mechanical, and both need the blower assembly examined rather than the electronics.
Most common causes
- An object drawn into or dropped into the blower wheel.
- Seized motor bearings.
- A blower wheel jammed against its housing after coming loose on the shaft.
- Debris packed into the wheel to the point where it cannot turn.
- Physical damage to the wheel or housing.
- A failed motor control module producing a false locked rotor indication.
How to diagnose it
- Note whether the blower has been making unusual noise recently. Bearings that seize usually complain first.
- Note whether anything has been stored on or near the furnace, or whether work has been done nearby that could have dropped material into the cabinet.
- Do not attempt to turn the wheel by hand to check. The blower compartment on a variable-speed furnace is not a safe place to reach into, and the motor electronics hold charge after power is removed.
- Turn the furnace off at its service switch and leave it off until it is looked at.
How to fix it
- Switch the furnace off and call a technician. A stalled motor under repeated start attempts is being damaged each time.
- This is entirely technician work — high voltage, stored charge in the motor module, and a mechanical assembly that needs correct reassembly.
- Have the cause of the jam established rather than just cleared. Something that got into the blower once can get in again if there is a gap in the cabinet or ductwork.
- Do not repeatedly reset the furnace hoping it will free itself. Each attempt heats a motor that cannot turn.
Notes
Of the blower codes, this is the one where continuing to reset the furnace does the most damage. A motor that will not turn is converting all its power into heat in a stationary winding.