Rinnai · Tankless Water Heater Critical

72 — Flame Sensor Fault

Flame sensing circuit reporting flame when there shouldn't be one, or behaving erratically. Safety-critical.

Keep using it?
No — service first
DIY difficulty
Hard
Verified against
rinnai.us ↗

How to fix it

  1. Power down unit.
  2. Remove and inspect flame rod — clean with fine emery cloth.
  3. Inspect flame rod wiring and porcelain insulator for cracks or moisture.
  4. Verify proper chassis ground / bonding.
  5. Replace flame rod if cracked or fouled.
  6. Replace PCB if sensor and wiring test good.

Why it happens

Typical signs: Code 72 with unit off · Unit won't start a heating cycle · Repeated lockout

  • Flame rod shorted to chassis Sensor or wire grounding out.
  • Moisture on flame rod or wiring Condensation causing false signal.
  • Failed PCB sense input Board misreading flame state.
  • Carbon bridge on flame rod Conductive carbon path to ground.

Parts you may need

If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.

More Rinnai tankless water heater codes

02 02 — Power Interruption / Memory Loss 03 03 — Loss of Communication / EZConnect Issue 10 10 — Air Supply or Exhaust Blockage 11 11 — No Ignition 12 12 — Flame Failure 14 14 — Thermal Fuse / Overheat Cutoff

All Rinnai codes → All tankless water heater codes →

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: rinnai.us.