Rinnai Error Code 12: Flame Failure

The Short Answer: Rinnai code 12 means the flame went out after ignition, or the flame rod stopped detecting it. Rinnai’s documents call it “Flame Failure” (RL series) or “No Flame” (SENSEI). Confirm the gas valve to the heater is open, other gas appliances work, and a propane tank is not empty. Restart once. A code 12 that returns needs a technician to test gas pressure, the flame rod, and the vent.

Code 11 or code 12?

The difference matters. Code 11 means the heater never proved a flame during the ignition attempt (see Rinnai code 11). Code 12 means it had flame, then lost it. With a 12, ignition parts usually worked; the question is why the flame could not stay stable. Rinnai’s RL75i/RL94i data sheet (opens in a new tab) lists gas supply, gas type, line sizing, air in the gas line, flame-rod condition and wiring, venting, and moisture or debris on the burner surface as the areas to check.

What you can check

1. The gas valve at the heater should be open (handle in line with the pipe).
2. Other gas appliances. If the range or furnace also struggles, the problem is the supply, not the water heater.
3. Propane. Check the tank gauge. A tank that is nearly empty, or a regulator icing up in cold weather, can let the heater light and then starve it.
4. The vent terminal. Look for snow, a nest, or a damaged cap from the ground.
5. Restart once by turning the controller off and on. Note whether the code returns right away or only during long or high-demand draws.

That last detail helps the technician. A code 12 that appears only on the coldest mornings, or only when the furnace runs at the same time, points to gas supply under load.

Why gas supply is the common thread

A tankless burner draws far more gas at full fire than at low fire. Rinnai’s professional troubleshooting guide says the minimum inlet pressure on the rating plate must hold with every gas appliance in the building at high fire, and it sets limits on how much the pressure may drop. A gas line or regulator that was sized for an old tank heater can pass a light-load test and still fail when the tankless ramps up. Correcting that is gas piping work, not a part in the heater.

What the technician tests

• Inlet gas pressure at the heater with all appliances running, compared to the rating plate
• Whether the gas type and the heater’s gas-type setting match
• Flame rod tightness, wiring, and carbon buildup, plus the flame signal
• Vent joints, total length, and terminal clearances (exhaust drawn back into the intake can cause incomplete combustion and a 12)
• Moisture or debris on the burner, and the gas valve if everything else checks out

Most fixes are minor: cleaning or replacing the flame rod, correcting a vent issue, or resizing a regulator or gas line. A gas valve or control board is a larger part repair, but code 12 alone is rarely a reason to replace the heater.

See also: code 10 air or exhaust blockage, a Rinnai that goes cold after a few minutes, and Rheem tankless code 12, which follows similar logic.

Frequently Asked Questions

Q: Why does my Rinnai show code 12 after a new propane delivery? A: Air can enter the line when a propane tank runs empty. Rinnai’s manuals list bleeding air from the gas line as a fix, but that is work for the propane supplier or a gas technician. Do not loosen fittings yourself.

Q: Is it safe to keep resetting code 12? A: One restart is reasonable after you have confirmed gas supply. Repeated resets do not fix the cause and add failed ignition attempts. If it returns, leave the heater off and book a service visit.