Rheem Red Light Flashing 9 Times: Chamber Sensor

The Short Answer: On Rheem atmospheric gas water heaters, nine red flashes every three seconds means a chamber sensor failure. On the larger 75- and 98-gallon models Rheem adds “or bimetal switch activated.” The sensor watches the combustion chamber for abnormal heat, which usually points to restricted combustion air, a blocked flame arrestor, or a venting problem. Rheem’s instruction is to contact a qualified technician.

What the chamber sensor does

Rheem’s flammable-vapor-resistant gas heaters have a sealed combustion chamber that takes in air through an arrestor at the base. A sensor in the chamber detects temperatures that suggest the flame is burning where it shouldn’t, for example when air is starved or something is burning on the arrestor. When it trips, the control shuts the heater down and flashes the code.

Rheem’s Use & Care manual AP23698 (opens in a new tab) lists:

The 75/98-gallon manual AP19928 (opens in a new tab) words 9 flashes as chamber sensor failure or bimetal switch activated. Power-vent Rheem heaters use a different code list. Check the label on your heater.

Why it happens

• Clogged air intake or flame arrestor. Lint, dust, and pet hair at the base starve the burner of air. This is common in laundry rooms and garages.
• Restricted space. Stored items packed around the heater, or a closet without enough air.
• Venting problems. A blocked or disconnected flue can cause abnormal combustion.
• Flammable vapors. Fumes burning on the arrestor can overheat the chamber.
• A faulty sensor or wiring, if none of the above is present.

What you can check safely

1. Count the flashes over two full cycles to confirm it’s 9, not 8 or 10.
2. Look at the floor and the air-intake area around the base of the heater for heavy lint or dust. Clean around it with a vacuum from the outside, but don’t remove any covers or the burner door.
3. Clear stored items away from the heater and make sure a closet door isn’t sealing it off from air.
4. Remove gasoline, paint, and solvents from the room.
5. Look at the flue pipe on top for an obvious disconnection, but don’t touch it if the heater has been running.

Then call a technician. Rheem’s manual lists this code as one for a qualified service technician, not an owner reset.

What the technician will check

The technician will inspect and clean the flame arrestor and combustion chamber, check the vent for proper draft, look for signs of flame rollout or scorching, and test the chamber sensor and its wiring. If they open the burner door, they replace its gasket, as Rheem specifies, so the chamber stays sealed. If the sensor tripped because of a real air or venting problem, that is fixed before the heater goes back in service.

Repair or replace

Cleaning the arrestor, correcting venting, or replacing the sensor are repairs. If the inspection finds a leaking tank or heavy damage in the combustion chamber, replacing the heater is usually the better choice.

Related: water heater chamber sensor failure and Rheem 7 flashes.

Frequently Asked Questions

Q: Can I reset a 9-flash code myself? A: Rheem directs owners to a qualified technician for this code. Resetting without finding the cause can put the heater back into unsafe operation.

Q: The flashes came back after a technician reset it. Why? A: The cause likely wasn’t fully resolved: a partly clogged arrestor, a venting problem, or a failing sensor. Ask for the arrestor and vent to be checked, and the sensor tested.