Fault Code 9

Aqua-Hot Furnace: “Fault Code 9”

warning RV furnace fault codes

Safety A flame-out followed by failed re-ignition can leave unburned diesel in the combustion chamber. Do not repeatedly retry ignition. Make sure the exhaust path is unobstructed before restarting, since incomplete combustion can produce carbon monoxide. Disconnect 120V AC before servicing.

What it means

Re-ignition failure. The burner lit and established a flame, the flame then went out during operation, and the burner failed to re-light on its re-ignition sequence.

Most likely causes

Dirty or fouled flame sensor lens so the controller cannot confirm the re-lit flame most likely
Obstructed or contaminated fuel nozzle interrupting fuel delivery common
Too much combustion air for the burner less common
Poor chassis ground connection less common

Safe checks you can do yourself

Inspect and gently clean the flame sensor lens so it can detect flame
Check the fuel nozzle for obstructions or contaminants
Verify the combustion-air setting is correct (not set too high)
Confirm the chassis ground connection is clean and secure

How to reset / clear Fault Code 9

Clean the flame sensor lens and clear any fuel/air issue first
Turn the diesel-burner switch OFF for about 30 seconds
Turn the switch back ON to clear the fault and restart

When to call a technician

If cleaning the flame sensor and checking the nozzle, combustion air and ground do not resolve repeated flame-outs, have the burner serviced or call Aqua-Hot Technical Support at 1-800-685-4298.

Applies to

Aqua-Hot 375-D02 · AHE-375-D02

FAQ

What does Aqua-Hot Furnace code "Fault Code 9" mean?

Re-ignition failure. The burner lit and established a flame, the flame then went out during operation, and the burner failed to re-light on its re-ignition sequence.

How do I reset Aqua-Hot Furnace Fault Code 9?

Clean the flame sensor lens and clear any fuel/air issue first Turn the diesel-burner switch OFF for about 30 seconds Turn the switch back ON to clear the fault and restart

What is the most common cause of Fault Code 9 on a Aqua-Hot Furnace?

The most likely cause is dirty or fouled flame sensor lens so the controller cannot confirm the re-lit flame.

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Sources

Aqua-Hot 375-D02 Hydronic Heating System Service Manual (Rev. E) — Section 9, Fault Code 9, p.95 (Aqua-Hot Heating Systems Inc.)

Note: Specific to the 375-D02 controller; blink assignments vary across Aqua-Hot models.

● Last verified 2026-06-29 · compiled from manufacturer documentation. Codes can vary by model year — always cross-check your unit’s manual.