Goodman GMVM970803BN Error Code b4: Blower Motor Current Trip or Lost Rotor

What Does Code b4 Mean?

The variable-speed ECM blower in the GMVM970803BN continuously monitors its own current draw and how well the rotor is tracking the electrical signals driving it. When the motor sees current spike to a dangerous level, or detects that the rotor is no longer following those signals (a loss of rotor control), it shuts itself down to avoid damage. The Integrated Control Module reports this as a b4 and stops the furnace, so unlike the low-severity b3 you get no heat at all.

It helps to read b4 against its neighbors on this board. A b3 is the motor merely limiting its output while still running; a b4 is a protective full shutdown after a current or rotor-tracking event. If the rotor cannot turn at all and the motor fails to start across 40 consecutive attempts, the board escalates to a b5 locked-rotor lockout. A shutdown caused by out-of-range line voltage or an over-temperature power module is a b6 instead. The common thread with b3 and b9 is that a severe, sudden airflow restriction can spike the motor's current enough to trip b4.

That overlap is why the homeowner-safe first checks are the same airflow items: filter, registers, and any obvious blockage right at the furnace's air inlet or outlet. Anything deeper, opening the blower housing, freeing a loose wheel, judging bearing wear, or touching wiring and the control board, is a technician task and is not covered by the DIY steps below.

What You'll Notice

• The blower and furnace are stopped and no warm air is being delivered
• The ComfortNet thermostat shows a Call for Service icon and scrolls Check Furnace
• The dual 7-segment display shows b4
• You may have heard the blower attempt to start and then abruptly cut out
• A heavily clogged filter or return grille, or an obvious object blocking an air opening

Common Causes

How to Fix It: Clear sudden airflow blockages that can trip the blower

What You'll Need

• Correct-size replacement air filter 🛒 Find at FiltersFast · 🛒 Find at Amazon
• Flashlight

Steps

1. Shut off power and gas first Turn off the furnace at its dedicated switch or the breaker, and shut off the gas supply at the manual valve on the gas line to the furnace. If you smell gas, leave immediately and call your gas company. Do not proceed until both power and gas are off.
2. Inspect and replace the air filter Pull the filter from the return duct or filter slot and check it against a light. If it is heavily clogged, replace it with the correct size on the frame, arrow pointing toward the furnace. A suddenly or severely blocked filter can create enough back-pressure to spike the motor's current and trip a b4.
3. Open all supply and return registers Confirm every supply and return register in the home is fully open and not covered by furniture, rugs, or curtains. A sudden restriction across the duct system raises the load on the blower.
4. Check the furnace air inlet and outlet for obvious blockage With a flashlight, look at the return opening and the supply plenum area of the furnace for anything that has fallen in or is covering the opening, and remove obvious loose obstructions. Do NOT remove the blower access panel or reach into the blower housing, and do not attempt any wiring, wheel, or bearing work.

When to Call a Professional

Contact a licensed HVAC technician if:

• The b4 returns after the filter, registers, and air openings have been checked and cleared
• There is grinding, scraping, or rattling coming from the blower area
• The blower wheel is visibly loose on its shaft or there is debris inside the housing
• The motor hums or tries to start but will not spin

✓ Verified against manufacturer service manual — March 2026