What causes Rheem Tankless Water Heater 12?

The gas shutoff valve to the heater isn't fully open, or gas service was interrupted. Propane running low (LP models). Air in the gas line, common after a new install or after the gas was shut off. Gas pressure dropping when the heater fires at full rate, often from an undersized gas line, meter or regulator, or another gas appliance running at the same time. Wrong gas type for the unit. Vent pipes disconnected, or the intake or exhaust termination blocked. A clogged gas inlet filter (or air intake filter on IKONIC models). Power supply not properly grounded. On the technician side: a dirty or failing flame rod, DIP switches not set for the venting and altitude, a bad gas valve solenoid, or a failed control board.

How do I fix Rheem Tankless Water Heater 12?

Reset the way Rheem's Use and Care manual says: close the hot water faucet, turn the remote control off, wait about 5 minutes, turn it back on and open the hot water faucet again. If 12 is still showing, close the faucet and turn the remote off before checking anything. Make sure the gas shutoff valve to the water heater is fully open, and check that your other gas appliances work. If none do, contact your gas utility. On propane units, check the tank level and refill or swap the cylinder if it's low. Notice whether 12 shows up mainly when other gas appliances (furnace, range, dryer) are running or when several hot water taps are open. Rheem says static pressure can look normal while pressure drops under load, which a technician checks with a dynamic pressure test. Look at the vent termination outside (intake and exhaust on direct-vent units) and clear leaves, snow or nests. Don't take apart vent connectors yourself. If the heater was just installed or the gas was recently off, have the installer bleed air from the gas line and confirm the gas type and line sizing. If 12 returns, note the code, turn the remote off and call Rheem customer service or a licensed technician. Rheem's manual says not to attempt repairs to gas piping, burners, vent connectors or safety devices yourself.

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

Code 12 keeps coming back after the gas valve, propane level and vents check out. Measuring static and dynamic gas pressure at the heater. Rheem's manuals give 4.0 to 10.5 inches water column for natural gas and 8.0 to 13.0 for propane, with dynamic pressure checked at maximum combustion. Cleaning the gas inlet filter, and on IKONIC units the air intake filter. Checking the flame rod signal and cleaning or replacing the rod. Rheem's IKONIC manual says the gasket must be replaced with a Rheem-approved one whenever the flame rod or igniter rod is removed. Testing the gas valve solenoid resistances and voltages and, if those fail, replacing the gas valve or control board as Rheem's flowchart directs. You smell gas at any point. Leave the area and call your gas company from outside.

Code: 12🚿 Rheem Tankless Water Heater

Rheem Tankless Water Heater Code 12 (A012): Flame Lit, Then Went Out (Meaning, Causes, Fixes)

Q: What does Rheem Tankless Water Heater 12 mean?

Code 12 on a Rheem tankless gas water heater means the burner lit but the flame went out during operation, several times in a row. Rheem's manuals point to the same homeowner checks as code 11: gas valve not fully open, low propane, air in the gas line, low gas pressure from an undersized line, the wrong gas type, or blocked or badly installed venting. Reset the unit the way the manual says, check those, and call a qualified gas technician if 12 comes back. Rheem's High and Super High Efficiency tankless troubleshooting manual defines error 12 as Detected Presence of Flame, Then Lost It. When the heater loses flame during combustion it shuts off the gas, tries to restart and counts the flame losses; after 3 in a row it shows 12 on the built-in display or the remote. On Rheem IKONIC models with the newer digital user interface the same fault shows as A012, and the IKONIC service manual says those units count 15 consecutive flame losses before posting it. That's the difference from code 11 (A011), where the burner never lights at all. Rheem's Use and Care error-code guide lists 11 and 12 together because the owner checks are the same. For technicians, Rheem's diagnostic items are gas supply pressure (static and dynamic), the gas inlet filter, venting, the flame rod, and the gas valve solenoids, plus DIP switch settings for venting and altitude on the models the troubleshooting manual covers, and the air filter, gas-air ratio valve and solenoid valve on IKONIC units.

Reviewed 2026-10-22 · Based on 3 manufacturer & service sources

What the code means

Rheem's High and Super High Efficiency tankless troubleshooting manual defines error 12 as Detected Presence of Flame, Then Lost It. When the heater loses flame during combustion it shuts off the gas, tries to restart and counts the flame losses; after 3 in a row it shows 12 on the built-in display or the remote. On Rheem IKONIC models with the newer digital user interface the same fault shows as A012, and the IKONIC service manual says those units count 15 consecutive flame losses before posting it. That's the difference from code 11 (A011), where the burner never lights at all. Rheem's Use and Care error-code guide lists 11 and 12 together because the owner checks are the same. For technicians, Rheem's diagnostic items are gas supply pressure (static and dynamic), the gas inlet filter, venting, the flame rod, and the gas valve solenoids, plus DIP switch settings for venting and altitude on the models the troubleshooting manual covers, and the air filter, gas-air ratio valve and solenoid valve on IKONIC units.

Likely causes

The gas shutoff valve to the heater isn't fully open, or gas service was interrupted
Propane running low (LP models)
Air in the gas line, common after a new install or after the gas was shut off
Gas pressure dropping when the heater fires at full rate, often from an undersized gas line, meter or regulator, or another gas appliance running at the same time
Wrong gas type for the unit
Vent pipes disconnected, or the intake or exhaust termination blocked
A clogged gas inlet filter (or air intake filter on IKONIC models)
Power supply not properly grounded
On the technician side: a dirty or failing flame rod, DIP switches not set for the venting and altitude, a bad gas valve solenoid, or a failed control board

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.

Reset the way Rheem's Use and Care manual says: close the hot water faucet, turn the remote control off, wait about 5 minutes, turn it back on and open the hot water faucet again.
If 12 is still showing, close the faucet and turn the remote off before checking anything.
Make sure the gas shutoff valve to the water heater is fully open, and check that your other gas appliances work. If none do, contact your gas utility.
On propane units, check the tank level and refill or swap the cylinder if it's low.
Notice whether 12 shows up mainly when other gas appliances (furnace, range, dryer) are running or when several hot water taps are open. Rheem says static pressure can look normal while pressure drops under load, which a technician checks with a dynamic pressure test.
Look at the vent termination outside (intake and exhaust on direct-vent units) and clear leaves, snow or nests. Don't take apart vent connectors yourself.
If the heater was just installed or the gas was recently off, have the installer bleed air from the gas line and confirm the gas type and line sizing.
If 12 returns, note the code, turn the remote off and call Rheem customer service or a licensed technician. Rheem's manual says not to attempt repairs to gas piping, burners, vent connectors or safety devices yourself.

🛠 When to call a pro

Code 12 keeps coming back after the gas valve, propane level and vents check out.
Measuring static and dynamic gas pressure at the heater. Rheem's manuals give 4.0 to 10.5 inches water column for natural gas and 8.0 to 13.0 for propane, with dynamic pressure checked at maximum combustion.
Cleaning the gas inlet filter, and on IKONIC units the air intake filter.
Checking the flame rod signal and cleaning or replacing the rod. Rheem's IKONIC manual says the gasket must be replaced with a Rheem-approved one whenever the flame rod or igniter rod is removed.
Testing the gas valve solenoid resistances and voltages and, if those fail, replacing the gas valve or control board as Rheem's flowchart directs.
You smell gas at any point. Leave the area and call your gas company from outside.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Combustible gas leak detector

Handy 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 units, so a low tank doesn't keep tripping code 12.

[affiliate link slot: AFFILIATE_SLOT_PROPANE_GAUGE]

⚙

Rheem tankless flame rod (exact part for your model)

Technician part only. Rheem's manual says to refer burner and safety-device repairs to qualified service personnel.

[affiliate link slot: AFFILIATE_SLOT_TANKLESS_IGNITER]

Sources

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

Rheem Tankless Water Heater Code 12 (A012): Flame Lit, Then Went Out (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

Rheem tankless flame rod (exact part for your model)

Related codes

Sources