Goodman furnace b4: Blower motor current trip or lost rotor
b4 means the motor either drew more current than it is allowed or lost track of its own rotor position. Both are the motor's electronics protecting the motor.
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
An ECM motor controls itself by knowing exactly where its rotor is at every moment. Two related failures are grouped under b4: a current trip, where the motor drew more than its limit and shut down, and a lost rotor, where the electronics could no longer determine the rotor's position and stopped rather than driving blindly. Both indicate the motor was unable to do what was asked under the conditions present. Mechanical resistance is the usual reason — anything making the wheel harder to turn produces higher current — but electrical supply problems produce the same result, because a motor at low voltage draws more current for the same work.
Most common causes
- A blower wheel obstructed by debris, or rubbing against the housing.
- Failing motor bearings, increasing the effort needed to turn.
- A blower wheel loose on the shaft or out of balance.
- Low supply voltage, raising current draw for the same output.
- A severely restricted duct system, though this more often produces b3.
- A failing motor control module.
- Something drawn into the blower compartment and caught in the wheel.
How to diagnose it
- Listen to the blower. Rubbing, grinding, rattling, or a wobble in the sound points at the wheel or the bearings.
- Note whether the fault is repeatable at a particular speed. A wheel that is out of balance often causes trouble at one end of the range.
- Note whether it started suddenly. A sudden onset suggests an obstruction; a gradual one suggests bearings.
- Check whether the house has had voltage problems, or whether the fault coincides with other electrical symptoms.
How to fix it
- This is technician work. Removing a blower assembly means working in a compartment with high voltage wiring and an ECM module that holds charge after power is off.
- Have the wheel and bearings checked before the motor is condemned. An obstruction or a loose wheel is a far cheaper repair than a motor.
- If supply voltage is low, that needs addressing separately — a new motor into a low voltage supply will trip in the same way.
- Do not keep resetting the furnace to get another cycle. Repeated current trips heat the motor electronics.
Notes
The lost rotor half of this code is worth understanding: an ECM motor stops rather than continuing when it loses positional tracking. That is a deliberate design choice, and it means b4 sometimes appears with no mechanical fault at all — the motor lost synchronisation and shut down safely.
Related
All codes for this manufacturer
Goodman and Amana furnace fault codes — every code, with what differs between models.