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
media.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 Code 02 — No Burner Operation During Freeze Protection 03 Code 03 — Power Interruption During Bath Fill 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 →

Frequently asked questions

What does the 72 code mean on a Rinnai tankless water heater?

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

What causes the 72 code on a Rinnai tankless water heater?

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.

Is it safe to keep using a Rinnai tankless water heater with a 72 error?

No — service first. This is rated a critical-severity fault.

How do I fix 72 on a Rinnai tankless water heater?

Power down unit. Remove and inspect flame rod — clean with fine emery cloth. Inspect flame rod wiring and porcelain insulator for cracks or moisture.

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: media.rinnai.us.