Why does my Rheem show code 12 only sometimes?

Intermittent code 12 often lines up with other gas appliances running or with windy weather. Note when it happens and tell your technician.

Rheem Tankless Code 12 (Flame Failure): What to Check

Where code 11 means the burner never lit, code 12 means it lit and then went out. The unit shuts off the gas and displays the code.

Code 12 is often intermittent. Pay attention to when it happens, because the timing points to the cause.

What to check first

Does code 12 appear when the furnace, dryer or range is also running? That points to gas supply.
Check the vent terminal outside, especially on windy days or after snow.
Confirm the gas valve to the heater is fully open and, on propane, that the tank has gas.
Reset the unit at the controller and run hot water.

Common causes

Gas pressure drops under load

If the gas line is undersized, pressure can sag when several appliances fire at once. The flame becomes unstable and goes out.

Venting and wind

A partly blocked vent or a terminal exposed to strong wind can disturb the flame.

Flame rod

Carbon or buildup on the flame rod can make the unit lose the flame signal even when the flame is burning. Cleaning or replacing it is a technician job.

When to call a technician

If code 12 keeps returning, a technician should measure gas pressure with everything running and service the flame rod.

Other Rheem codes

Code	Meaning
00	Continuous combustion for more than one hour
03	Communication failure between units, remote or manifold controller
05	Improper combustion detected by the flame rod (warning)
10	Air supply or exhaust blockage (hard lockout)
11	Ignition failure
12	Flame failure
13	Poor combustion detected by the flame rod (indoor units)
14	Overheat safety tripped
15	Heat exchanger or hot water temperature too high (boiling safety)
16	Outlet water temperature above the set point (over temperature warning)
21	Air intake filter switch fault (earlier models)
24	Remote control buttons held too long, or operational switch fault
29	Condensate not draining (condensing models) or heat exchanger temperature too low
31	Inlet water temperature sensor (thermistor) fault
32	Heat exchanger thermistor fault
33	Outlet (outgoing water) thermistor fault
34	Ambient or combustion air temperature sensor fault
35	Improper thermistor connection
51	Gas valve fault: flame detected after shutdown (current) / gas inlet solenoid valve fault (earlier)
52	Modulating gas valve (PGFR) fault
61	Blower (combustion fan) motor fault
65	Water volume control valve fault
66	Water bypass valve fault
71	Gas inlet solenoid valve fault
72	Flame rod fault: flame sensed before ignition
76	Remote control communication fault
79	Blower (fan) motor current fault
80	Gas cut-off failure: flame detected after the unit turned off
82	Control board program chip fault (current) / gas type data failure (earlier)
90	Vent blockage detected before ignition
92	Neutralizer needs replacing soon (warning, condensing models)
93	Neutralizer must be replaced; unit will not operate (condensing models)
1L	Lime (scale) buildup detected in the heat exchanger
99	Vent blockage detected after shutdown / blocked flue (hard lockout)
P1	Not enough water flow to fire the burner (warning)

Rheem tankless code 12: flame failure, causes and fixes