Rheem tankless gas water heater code 72: meaning and exact next steps

Model-specific

What is true of this model and not of its siblings

A false reading

The control uses the flame rod to check for flame after the igniter fires. A reading of flame before the igniter has fired means the sensor or board is wrong, or a flame is present when it should not be.

Safety first

If a flame is genuinely present at the wrong time, treat it as a gas valve fault and turn off the gas. Otherwise a sensor or wiring fault is likely.

Technician work

Testing the flame rod, the wiring and the board needs a technician.

A flame that is not there

The control looks for flame only after it fires the igniter. If the rod reports flame before that, the sensing circuit is wrong, perhaps shorted by moisture or damaged, or a flame truly exists where it should not.

Moisture

Condensation or a leak can bridge the flame rod to ground and mimic a flame. Water marks near the burner are worth reporting.

Safety response

Rheem's row suggests turning off the gas at the shut-off valve, because a real flame at the wrong time is a gas control fault. A technician tests the rod and the valve.

Questions for the technician

Ask whether the flame rod and its wiring were tested, whether any moisture was found and whether the gas valve passed a leak-by test. A flame rod that reads flame in standby is either wet, shorted or reading a real flame, and each has a different repair.

Keep the area dry

Wipe away any condensation or spills under the unit, but do not spray the heater. If water appears repeatedly, find the source before the repair, since a new flame rod will fail the same way if moisture reaches it.

Why timing matters

The control expects no flame before the igniter fires and a flame shortly after. A flame signal at the wrong time means either a real fire in the combustion chamber, a stuck gas valve or a sensing fault. Because the first two are dangerous, the control locks out rather than trying again.

What to tell the technician

Tell the technician what you heard and saw: whether the burner lit, whether there was a click, a smell or a sound of rushing gas, and whether the code came on the first demand of the day. Those details narrow the diagnosis quickly.