4 Flashes
AO Smith Water Heater
Severity: ModerateWhat Does This Error Mean?
AO Smith gas water heaters use a small status LED on the gas control valve.
Four flashes in a repeating pattern means the vent pressure switch did not close when the inducer fan started.
The control will not light the burner without that confirmation — that prevents firing with no draft.
Cause is usually a blocked vent, a stuck inducer, or a failed switch.
Affected Models
- AO Smith Vertex
- AO Smith ProMax
- AO Smith Signature Premier
- AO Smith Voltex hybrid (gas backup)
- AO Smith FCG series
Common Causes
- Vent pipe blocked by a bird's nest, wasp nest, or debris
- Inducer fan failed or running too slowly
- Pressure switch tubing kinked or disconnected
- Pressure switch itself failed
- Outdoor termination iced over in winter
- Vent system installed too long or with too many elbows
How to Fix It
-
Power off the gas and electrical supply.
Turn the gas valve to OFF.
Switch off the dedicated circuit.
Wait 5 minutes for the inducer to fully stop before opening any panel. -
Inspect the vent termination.
Find where the vent exits the building.
Look for nests, leaves, snow, or ice covering the screen.
Clear anything blocking the path.
Many recurring 4-flash codes turn out to be a wasp nest from the previous summer. -
Listen for the inducer.
Restore power but leave the gas off.
Trigger a heat call (open a hot tap to drop tank temperature).
The inducer fan should spin up audibly within a few seconds.
Silent or a faint hum followed by silence = inducer failed. -
Check the pressure switch tubing.
The pressure switch is a small disc near the inducer with a thin clear or black tube running back to the inducer housing.
Disconnect the tubing.
Blow into it — you should hear or feel the switch click.
No click means the switch is dead. -
Replace the pressure switch.
Genuine AO Smith pressure switches are 30-90 USD.
Two wire terminals and one tubing connection.
Match the spec on the existing switch — pressure ratings differ between models, and a wrong-rated switch will trip just as readily as a dead one. -
Check vent length and slope.
AO Smith specifies a maximum equivalent vent length and a minimum slope back toward the unit so condensate drains.
If the install is borderline, condensate pooling in a horizontal run can block the vent and cause repeating 4-flash codes.
Reroute or shorten if possible. -
Replace a failed inducer.
If the fan is silent or runs slowly, the inducer motor or its capacitor failed.
Inducer assemblies are 100-250 USD depending on model.
Two screws, two electrical connections, and one vent connection — about a 30-minute job.
Frequently Asked Questions
Is 4 flashes safe to keep using?
The unit will not fire with 4 flashes active, so there is no carbon monoxide risk from this code itself.
The risk is bypassing the switch to make the heater run anyway — that is exactly the scenario the protection exists to prevent.