What causes Rinnai Tankless Water Heater 12?

Gas turned off at the water heater, the gas meter or the propane cylinder, or an empty propane tank. These are on Rinnai's own code 12 remedy list. Inlet gas pressure below the rating plate minimum when every gas water heater at the location fires on high, usually an undersized gas line, meter, regulator or tank, or dirt in the gas system. Wrong gas type, air still in the gas lines, or DIP switches and programming parameters set wrong. Venting problems: loose joints, wrong fittings or materials, too long an equivalent vent length, vent terminal clearances not met, or concentric or common vent pieces not fully pushed in, which Rinnai says can recirculate exhaust. A blocked flue outlet, or debris or insects in the fan blower wheel or intake air chamber. A loose spark electrode or flame rod, carbon build-up on the flame rod, or loose or damaged flame rod wiring at the rod or PC board. A cracked or heat-damaged silicone sleeve on the flame rod. Poor grounding, corrosion at ground connections, or reversed polarity on the electrical circuit. Moisture in wiring harness connections, or a DC short in a component (on SENSEI models this shows up as an immediate code 12). Condensation or debris on the burner surface, a condensation collar not installed properly, or a faulty gas valve or gas solenoid valves. A two-stage propane regulator mounted closer than six feet to the heater, or one that lets pressure bleed through above the heater's maximum.

How do I fix Rinnai Tankless Water Heater 12?

Check that the gas valve on the line into the water heater is open and that your other gas appliances work. If none of them do, call your gas utility. On propane, check the tank gauge and refill or swap the tank if it's low or empty. Look at the vent termination outside. Clear away snow, leaves, nests or anything else blocking the flue outlet or air intake, without taking the vent apart. Close the hot water tap, then open it again so the heater runs another ignition cycle. Rinnai's manual says the code stops flashing once the problem is fixed and the heater supplies hot water. If the heater was just installed, moved, or had vent work done, ask the installer to recheck the vent joints, vent length and terminal clearances, since Rinnai ties all of these to code 12. If 12 keeps returning, stop retrying and call a licensed professional. Rinnai's manual marks the code 12 remedies as licensed professional only. If you smell gas at any point, leave the house and call your gas company from outside. Don't flip switches or try to relight anything.

When should I call a pro for Rinnai Tankless Water Heater 12?

Code 12 returns after the gas supply, propane level and vent outlet check out. Measuring inlet gas pressure with every gas water heater at the location firing on high. Rinnai says the drop on a properly sized black iron system should be no more than 0.3 inches WC on natural gas or 0.5 inches on propane, and inlet pressure should never exceed 10 inches WC on natural gas or 13.5 inches on propane when the heater shuts off. Separating and checking the depth of each concentric or common vent joint, and confirming the vent length and materials meet Rinnai's or the vent maker's specs. Tightening or replacing the spark electrode and flame rod with a new gasket (Rinnai specifies a 0.138 inch, 3.5 mm electrode gap), and inspecting the flame rod wiring and silicone sleeve. Checking ground connections and polarity, drying out any damp harness connections, and tracking down a DC short by unplugging one component at a time. Inspecting the fan wheel, burners and gas valve, or isolating the heater from an MSA or MSB multi-unit system, which Rinnai says to do while troubleshooting. The heater is under warranty. Contact Rinnai or a Rinnai-authorized professional instead of opening it yourself.

Code: 12🚿 Rinnai Tankless Water Heater

Rinnai Tankless Error Code 12: Flame Failure, No Flame Detected (Meaning, Causes, Fixes)

Q: What does Rinnai Tankless Water Heater 12 mean?

Code 12 on a Rinnai tankless water heater is a flame failure: the heater isn't seeing a proper flame at the burner, so it shuts down and you lose hot water. Rinnai's own first checks are simple: make sure the gas is on at the water heater, the gas meter or the propane tank, that a propane tank actually has gas, and that nothing is blocking the flue outlet. If those are fine and 12 keeps coming back, Rinnai marks the rest (gas pressure and line sizing, venting, flame rod, wiring, gas valve) as licensed professional only. Rinnai's Troubleshooting Tankless Water Heater Diagnostic Codes manual titles code 12 Flame Failure and applies it to SENSEI, SENSEI RX/CX and non-SENSEI models; the VA Indoor manual also calls it Flame Failure, and the V53De Installation and Operation Manual lists it as No Flame. Rinnai's remedy list overlaps heavily with code 11 (No Ignition): gas supply, gas pressure and line sizing, and the flame rod and spark electrode. Rinnai's manual says a loose flame rod or electrode can cause either code. What code 12 adds is a strong focus on venting and electrical faults. Rinnai says vent terminals that don't meet clearances, or concentric or common vent joints that aren't fully engaged, can recirculate combustion gases and cause incomplete combustion and a code 12. A cracked flame rod sleeve can let the spark find ground outside the combustion chamber, and moisture in wiring harness connections can cause a short, both of which Rinnai says can lead to code 12. On SENSEI models, Rinnai says a code 12 that appears immediately after water starts flowing, with no flame visible in the burner box, points to an electrical short in a component on the DC circuit, such as a wiring harness, the water flow control or the bypass valve. The code stops flashing once the problem is corrected and the heater supplies hot water.

Reviewed 2026-10-22 · Based on 5 manufacturer & service sources

What the code means

Rinnai's Troubleshooting Tankless Water Heater Diagnostic Codes manual titles code 12 Flame Failure and applies it to SENSEI, SENSEI RX/CX and non-SENSEI models; the VA Indoor manual also calls it Flame Failure, and the V53De Installation and Operation Manual lists it as No Flame. Rinnai's remedy list overlaps heavily with code 11 (No Ignition): gas supply, gas pressure and line sizing, and the flame rod and spark electrode. Rinnai's manual says a loose flame rod or electrode can cause either code. What code 12 adds is a strong focus on venting and electrical faults. Rinnai says vent terminals that don't meet clearances, or concentric or common vent joints that aren't fully engaged, can recirculate combustion gases and cause incomplete combustion and a code 12. A cracked flame rod sleeve can let the spark find ground outside the combustion chamber, and moisture in wiring harness connections can cause a short, both of which Rinnai says can lead to code 12. On SENSEI models, Rinnai says a code 12 that appears immediately after water starts flowing, with no flame visible in the burner box, points to an electrical short in a component on the DC circuit, such as a wiring harness, the water flow control or the bypass valve. The code stops flashing once the problem is corrected and the heater supplies hot water.

Likely causes

Gas turned off at the water heater, the gas meter or the propane cylinder, or an empty propane tank. These are on Rinnai's own code 12 remedy list.
Inlet gas pressure below the rating plate minimum when every gas water heater at the location fires on high, usually an undersized gas line, meter, regulator or tank, or dirt in the gas system
Wrong gas type, air still in the gas lines, or DIP switches and programming parameters set wrong
Venting problems: loose joints, wrong fittings or materials, too long an equivalent vent length, vent terminal clearances not met, or concentric or common vent pieces not fully pushed in, which Rinnai says can recirculate exhaust
A blocked flue outlet, or debris or insects in the fan blower wheel or intake air chamber
A loose spark electrode or flame rod, carbon build-up on the flame rod, or loose or damaged flame rod wiring at the rod or PC board
A cracked or heat-damaged silicone sleeve on the flame rod
Poor grounding, corrosion at ground connections, or reversed polarity on the electrical circuit
Moisture in wiring harness connections, or a DC short in a component (on SENSEI models this shows up as an immediate code 12)
Condensation or debris on the burner surface, a condensation collar not installed properly, or a faulty gas valve or gas solenoid valves
A two-stage propane regulator mounted closer than six feet to the heater, or one that lets pressure bleed through above the heater's maximum

Safe DIY checks

⚠ Before you start

Unplug the appliance (or switch off its breaker) and shut off the water supply valves before removing panels or disconnecting hoses. Keep towels handy for residual water.

Check that the gas valve on the line into the water heater is open and that your other gas appliances work. If none of them do, call your gas utility.
On propane, check the tank gauge and refill or swap the tank if it's low or empty.
Look at the vent termination outside. Clear away snow, leaves, nests or anything else blocking the flue outlet or air intake, without taking the vent apart.
Close the hot water tap, then open it again so the heater runs another ignition cycle. Rinnai's manual says the code stops flashing once the problem is fixed and the heater supplies hot water.
If the heater was just installed, moved, or had vent work done, ask the installer to recheck the vent joints, vent length and terminal clearances, since Rinnai ties all of these to code 12.
If 12 keeps returning, stop retrying and call a licensed professional. Rinnai's manual marks the code 12 remedies as licensed professional only.
If you smell gas at any point, leave the house and call your gas company from outside. Don't flip switches or try to relight anything.

🛠 When to call a pro

Code 12 returns after the gas supply, propane level and vent outlet check out.
Measuring inlet gas pressure with every gas water heater at the location firing on high. Rinnai says the drop on a properly sized black iron system should be no more than 0.3 inches WC on natural gas or 0.5 inches on propane, and inlet pressure should never exceed 10 inches WC on natural gas or 13.5 inches on propane when the heater shuts off.
Separating and checking the depth of each concentric or common vent joint, and confirming the vent length and materials meet Rinnai's or the vent maker's specs.
Tightening or replacing the spark electrode and flame rod with a new gasket (Rinnai specifies a 0.138 inch, 3.5 mm electrode gap), and inspecting the flame rod wiring and silicone sleeve.
Checking ground connections and polarity, drying out any damp harness connections, and tracking down a DC short by unplugging one component at a time.
Inspecting the fan wheel, burners and gas valve, or isolating the heater from an MSA or MSB multi-unit system, which Rinnai says to do while troubleshooting.
The heater is under warranty. Contact Rinnai or a Rinnai-authorized professional instead of opening it yourself.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Combustible gas leak detector

For peace of mind around a gas water heater. Any real gas smell means leave and call the gas company, not a DIY check.

[affiliate link slot: AFFILIATE_SLOT_GAS_LEAK_DETECTOR]

⚙

Propane tank gauge

For LP installations, so an empty tank doesn't show up as a flame failure.

[affiliate link slot: AFFILIATE_SLOT_PROPANE_GAUGE]

⚙

Rinnai flame rod, electrode and gasket (exact part for your model)

Technician part only. Rinnai marks code 12 remedies as licensed professional only.

[affiliate link slot: AFFILIATE_SLOT_TANKLESS_IGNITER]

Sources

Last reviewed 2026-10-22. Always confirm against the manual for your exact model number.

Rinnai Tankless Error Code 12: Flame Failure, No Flame Detected (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Combustible gas leak detector

Propane tank gauge

Rinnai flame rod, electrode and gasket (exact part for your model)

Related codes

Sources