5 Flashes
AO Smith Water Heater
Severity: CriticalWhat Does This Error Mean?
AO Smith status LED flashing 5 times indicates a blocked exhaust vent — the unit detected pressure or temperature signs that exhaust gas is not leaving as it should.
This is a serious code: continued operation with a blocked vent risks carbon monoxide spilling into the home.
Do not bypass the lockout.
Clear the vent first, then reset.
Affected Models
- AO Smith ProMax
- AO Smith Signature Premier
- AO Smith Vertex (gas backup)
- AO Smith FCG series
Common Causes
- Bird, wasp, or rodent nest at the outdoor vent termination
- Snow or ice covering a sidewall termination
- Disconnected or fallen vent pipe section
- Excessive vent length added during a remodel
- Negative pressure in the home (kitchen exhaust hood, dryer running) overpowering the vent
How to Fix It
-
Stop using the heater.
Do not attempt to relight while 5 flashes is showing.
Open windows nearby for ventilation.
If you have any reason to suspect carbon monoxide spill, leave the home and run a CO meter check on return. -
Inspect the vent termination.
Walk outside to where the vent ends.
Look up at the cap if it is on the roof, or at the sidewall hood if it is on a wall.
Birds nests are very common in spring — wasps in summer.
Clear by hand or with a brush. -
Look for fallen sections.
Especially in attics or unconditioned spaces, vent sections can come apart over years from heat cycles.
Trace the vent from the heater to the termination.
Any disconnected joint = exhaust dumping into the building, not outside. -
Clear winter ice.
Concentric vent caps can ice over in cold climates.
Pour warm water gently to clear.
If this is an annual problem, an extension or heated cap solves it for under 100 USD. -
Check for backdraft.
Run the kitchen range hood and bath fans.
If 5 flashes appears only when those run, the home has negative pressure stronger than the heater's draft.
A make-up air vent or a smaller range hood resolves the conflict. -
Reset and verify.
Once the vent is confirmed clear, cycle the gas valve OFF then to PILOT then to ON.
Run a hot tap to trigger a heat call.
Watch the LED — solid or 1-flash means normal.
If 5 flashes returns, do not use the heater and call a plumber.
Frequently Asked Questions
Is 5 flashes a CO emergency?
The heater is OFF when 5 flashes is active, so it is not actively spilling CO.
The point of the lockout is exactly to prevent that.
The emergency case is bypassing the lockout to keep using the heater with the vent still blocked — do not do that.