Rinnai Tankless Water Heater 12 — Flame Failure
Rinnai Tankless Water Heater · Error code 12
What 12 Means
The unit lit successfully but lost flame signal during operation. Indicates intermittent gas, combustion, or sensing problem.
Symptoms
- Hot water starts then goes cold
- Burner cycles on/off rapidly
- Code 12 mid-shower
Common Causes
Carbon or scale on flame sensor preventing flame detection.
Inlet pressure drops below spec when burner ramps up.
Line too small for BTU load over the run length.
Partially blocked intake/exhaust causing unstable combustion.
Solenoid not metering correctly at high fire.
Flame rectification requires good chassis ground.
How to Fix It
- Remove and clean the flame rod with fine emery cloth — do not over-polish.
- Verify chassis ground and that the unit is bonded properly.
- Have inlet gas pressure measured at high fire (dynamic) — must stay in spec.
- Confirm gas line sizing and length are correct per install manual.
- Inspect vent terminations for partial blockage.
- If flame rod and gas pressure are good, replace the gas valve assembly.
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
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.