Goodman furnace b6: Blower motor over or under voltage trip, or over temperature trip
b6 means the motor shut down because its supply voltage went outside the acceptable range, or because it got too hot. Two of these three causes are outside the furnace entirely.
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
The motor's electronics monitor their own supply voltage and temperature and stop the motor if either goes out of bounds. That groups three quite different situations under one code. Over and under voltage are supply problems — the furnace is being fed electricity outside the range the motor tolerates, which is a house wiring or utility issue rather than a furnace fault. Over temperature is usually about airflow and ambient conditions around the motor. The practical value of knowing this is that two thirds of the causes are not the furnace's fault, and replacing furnace parts will not address them.
Most common causes
- Supply voltage low, which can be a utility problem, an undersized circuit, or a loose connection.
- Supply voltage high, less common but produced by some generator and utility conditions.
- A loose or corroded connection in the furnace's high voltage supply, causing voltage to sag under load.
- The motor overheating because airflow is restricted, so it is working hard and being cooled poorly.
- High ambient temperature in the furnace space — a hot attic or a confined closet.
- A failing motor control module misreading its own conditions.
- Operating from a generator with poor voltage regulation.
How to diagnose it
- Note whether other appliances in the house show voltage symptoms — lights dimming when large loads start is a useful clue.
- Note whether the fault correlates with weather. Over temperature faults cluster in hot conditions, particularly in attic installations.
- Note whether the furnace has been running on a generator, which is a common and easily forgotten cause.
- Check the air filter and registers, since restricted airflow contributes to the over temperature half of this code.
- Note whether the fault appeared after electrical work in the house.
How to fix it
- Replace a restrictive filter and open closed registers, which addresses the airflow contribution to overheating.
- Supply voltage problems need an electrician, not a heating technician — the furnace is reporting a fault it did not cause.
- Have the furnace's high voltage connections checked for tightness if voltage sags under load.
- Improve ventilation in the furnace space if ambient temperature is the issue, particularly in attic installations.
- Do not have the motor replaced before the supply has been checked. A new motor in the same electrical conditions will trip in the same way.
Notes
This is the furnace code most likely to be reporting a problem elsewhere in the building. A technician who checks supply voltage before condemning the motor saves the cost of an expensive part that was never faulty.