Rinnai Water Heater Code 12: What It Means and How to Fix It

Last updated: October 5, 2026 · Codes covered: 12 · Sources

Verify in your manual: Rinnai's SENSEI N-Series manual calls code 12 'No Flame'; older Rinnai code tables say 'Flame Failure'. Checks vary by model.

Code 12 on a Rinnai tankless water heater means flame failure: the burner lit but the heater lost its flame, so it shut down. Common causes include a gas supply problem, a blocked vent, or a dirty flame sensor. You can safely check the gas supply, the power, and the outside vent yourself, but anything inside the heater is a job for a licensed technician.

What the 12 code means

Rinnai's code tables call code 12 "Flame Failure." The current SENSEI N-Series Installation and Operation Manual and tech sheet call it "No Flame." Either way, the heater didn't detect the flame it needed, so it stopped heating. Rinnai's code 12 checks include the flame rod, the sensor that detects the flame.

You may notice hot water that turns cold, or no hot water at all.

Look-alike codes that mean something different:

• Code 11 ("No Ignition") means the burner never lit at all. The first checks are similar.
• Code 10 points to an air supply or exhaust blockage (or a full condensate trap on condensing models). A blocked vent can cause code 12 too, so if you see both, start with the vent.

Rinnai's detailed checks vary by model and series (SENSEI, condensing, non-condensing, indoor, outdoor), so keep your model's Installation and Operation Manual handy.

Likely causes (most common first)

1. Gas supply problem. The manual gas valve under the heater is partly closed, the propane tank is low or empty, or gas is off at the meter. Rinnai lists these first for code 12.
2. Blocked flue outlet or vent. Snow, ice, leaves, or a nest at the exhaust or intake, or a vent cap that's damaged. Rinnai lists "check for obstructions in the flue outlet."
3. Gas pressure or line size problem. Rinnai lists gas line, meter, and regulator sizing, plus gas type and pressure, among its code 12 checks. Only a licensed gas pro can check these.
4. Air in the gas line, typically after a new install or gas work. Bleeding the line is an installer's job.
5. Venting installed wrong. Unapproved vent material, a vent run that's too long, or loose joints. Rinnai lists these as code 12 checks for its technicians.
6. Power or grounding issues. Rinnai lists loose power connections, voltage drops, and grounding. Make sure the heater is plugged straight into a working outlet.
7. Flame rod or internal fault. Carbon on the flame rod, a loose flame rod wire, debris on the burner, wiring harness problems, a gas valve fault, or a control board ground problem. All of these are inside the sealed unit and need a technician.

How to fix it, step by step

Do these in order and try a hot tap after each step. Stop if you smell gas.

1. Reset the heater.
   • Turn off all hot water taps.
   • Switch the heater off at the temperature controller's On/Off button, wait a few seconds, and switch it back on.
   • Open a hot tap. Rinnai notes that codes 11 and 12 reset once water flow stops for a few seconds and starts again. (That note comes from a bulletin for multi-unit systems, so check your manual for your model.)
2. Check the manual gas valve at the heater. It's usually right below the unit and should be fully open. Turn it by hand only, never with tools. If it won't turn, don't force it. Call a technician.
3. Check your other gas appliances. If the stove, dryer, or furnace also won't light, the problem is the gas supply. Call your gas utility or propane supplier.
4. Check the propane tank (LP systems). If the gauge is low, call your propane supplier to refill it and restore the supply.
5. Clear the vent outside. Look at the vent termination (or the front of the heater on outdoor models) and remove snow, ice, leaves, or debris. Rinnai says to keep the area around the flue terminal clear. Don't take the vent apart. If a blockage can't be cleared easily, call a pro.
6. Check the power. Make sure the heater is plugged in firmly and the breaker is on. Rinnai says not to use an extension cord or adapter plug.
7. Notice when it happens. Does code 12 show up when several hot taps or gas appliances run at the same time, in windy weather, or only in winter? Tell your technician. Those patterns help them check gas supply, venting, and pressure.
8. Stop and call a pro if code 12 keeps coming back. Gas pressure, gas line sizing, venting, the flame rod, the burner, wiring, and the control board all involve live gas or live voltage. Leave them to a licensed technician.

Test the part

The part most often tied to code 12 is the flame rod, along with the burner and gas valve. These are inside the sealed combustion area, and Rinnai's own flame rod check is a live test with the heater running. That's not a homeowner job. Here's what you can safely check.

1. Unplug the water heater and turn off the gas. Pull the power cord or switch off its breaker. Rinnai notes that the temperature controller doesn't cut the power. Then turn the manual gas valve below the heater to OFF, by hand only.
2. Inspect the power cord and outlet. Look for damage, scorching, or a loose plug.
3. Inspect the vent from the outside. Look along any vent pipe you can see for loose or separated joints, damage, or crushed sections. Don't take it apart. Loose or separated vent joints are a reason to call a pro right away.
4. Check the vent termination. The exhaust and intake openings should be clear and undamaged.
5. Look for water damage under or around the unit. Rinnai says not to use the heater if any part has been under water.
6. Don't remove the front panel to check the flame rod or burner. Leave that to a licensed technician.

When you're done, open the gas valve by hand and restore power, following the "Operating Instructions" label on the heater.

Safety notes

When to call a pro

Call a licensed plumber or gas technician, or Rinnai, if:

• Code 12 keeps coming back after a reset, with the gas on and the vent clear.
• It happens when several gas appliances run at once, which may point to gas line sizing or pressure.
• The heater was just installed, converted, or moved, or the gas was recently worked on.
• You see loose vent joints, scorching, or water damage.
• The heater is under warranty. Rinnai recommends licensed professionals for service, and changes to the appliance can void the warranty.

Rinnai Customer Care is at 1-800-621-9419, Monday to Friday, 8 a.m. to 8 p.m. ET (per Rinnai's FAQ). Have your model number and the code ready.

FAQ

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