Rinnai code 72: flame rod fault

The flame rod is a small metal probe that sits in the burner flame. A tiny electrical current passes through the flame, and that current tells the control board the burner is really lit. Code 72 means the board is not getting a reliable signal from it.

Rinnai makes several generations of tankless heaters (SENSEI, SENSEI RX/CX and older non-SENSEI models). The meaning below comes from Rinnai's own diagnostic code guide; the exact service steps differ slightly between generations.

Common causes

• Carbon buildup on the rod, which weakens the signal
• A rod that has shifted and no longer touches the flame
• Damaged wiring between the rod and the control board
• A leaking heat exchanger dripping water into the burner area, which Rinnai lists as something to rule out
• A control board fault, once the rod and wiring check out

What a technician does

Rinnai's guide calls for measuring the flame rod's output, which should hold at least 1 microamp. If it is low, the rod is cleaned or, more often, replaced along with its gasket. The technician also checks that the heat exchanger is not leaking.

How to reset it

1. Close all hot water taps.
2. Turn the water heater off at the temperature controller, wait about a minute, then turn it back on.
3. Open a hot tap and watch whether the code returns.

If the same code comes back, the cause is still there. Resetting over and over is not a repair.

Why carbon builds up

Carbon on the flame rod usually points to a burn that is not quite right, from restricted airflow, a dirty burner or venting problems. If the rod keeps fouling, ask the technician to look at combustion as well as replacing the part.

See the full list on our Rinnai error codes page.