72 Critical Hard — consider a tech

Rinnai Tankless Water Heater 72 — Flame Sensor Fault

Rinnai Tankless Water Heater · Error code 72

What 72 Means

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

Symptoms

  • Code 72 with unit off
  • Unit won't start a heating cycle
  • Repeated lockout

Common Causes

1
Flame rod shorted to chassis

Sensor or wire grounding out.

2
Moisture on flame rod or wiring

Condensation causing false signal.

3
Failed PCB sense input

Board misreading flame state.

4
Carbon bridge on flame rod

Conductive carbon path to ground.

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.

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.

Common replacement Flame sensor rod View on Amazon→

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 →

Verified against manufacturer documentation: rinnai.us

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.