Rinnai Code 12 is distinct from Code 11: the unit lit successfully but the flame dropped out while the water was being heated. The most common cause is a drop in gas supply pressure mid-cycle, often from competing appliances drawing gas simultaneously. A worn or cracked flame rod that works for initial ignition but cannot sustain a signal during operation is the second most frequent cause.
I usually see this code when someone's halfway through a shower and the water suddenly goes cold on them. Code 12 means the heater fired up fine but couldn't stay lit under load. Most of the time it's a gas supply that can't keep up when the furnace or something else kicks on, or a flame rod that's just grimy enough to lose its signal once things heat up.
Phillips #2 screwdriver, Scotch-Brite pad or fine steel wool (for flame rod cleaning)
What Does the 12 Code Mean?
When I'm out on a service call for Code 12, I'm looking for what changed after the fire started. We know the igniter and gas valve worked at first, so now I'm hunting for whatever's stealing the flame. Usually it's a pressure battle, either the gas supply's dropping out mid-cycle or the venting's pushing combustion gases back against the burner. Both are fixable, but you've gotta find which one first.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Gas supply pressure dropping below minimum during high-demand periods40%
Worn or cracked flame rod that can sustain ignition but loses signal under load24%
Code 12 flashes on the controller and the unit goes completely silent, no burner noise, no fan, nothing.
You get a solid 30 to 60 seconds of hot water in the shower, then it drops straight to freezing cold and won't recover no matter how long you wait.
The code shows up way more often when the dishwasher's running, the stove is going, or it's a cold night when the furnace keeps cycling.
You start noticing Code 11 popping up occasionally on the same unit too, which usually means the flame rod is wearing out and throwing whichever code fits that day's conditions.
Unit resets fine and even runs for a couple minutes, but give it a full shower cycle and it cuts out every single time.
Can you reset a Rinnai waterheater to clear the 12 code?
To clear the code, power the unit off using the controller button, wait about 30 seconds, then power it back on. Just keep in mind that Code 12 is rarely a fluke. If it keeps happening, your Rinnai is telling you that the combustion environment is not stable. A reset is just a temporary bandage for a supply or sensor issue.
Tools Required for Diagnosis
Phillips #2 screwdriverScotch-Brite pad or fine steel wool (for flame rod cleaning)Manometer or water column gauge (for gas pressure testing)5-gallon bucket and stopwatch, or inline flow meter (for GPM verification)Flashlight or headlampAdjustable wrench (for accessing gas inlet pressure tap)
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
Replacement Parts
If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.
Part Name
OEM Number
Estimated Price
Flame Rod (Flame Sensor)100074985 · $12–$25
100074985
$12 – $25
Gas Pressure Regulator (external)Varies by installation · $25–$80
Varies by installation
$25 – $80
Frequently Asked Questions
What is the difference between Rinnai Code 11 and Code 12?
I explain it like this: Code 11 is a no-start, Code 12 is a stall. With an 11, the unit never got the fire going at all. With a 12, the fire started but couldn't be sustained. That tells me to skip the igniter entirely and focus on things that affect a running flame, like gas volume dropping mid-cycle, vent backpressure building up, or a flame rod that holds its signal cold but loses it once it reaches operating temp.
Can I fix Code 12 myself or do I need to call a pro?
Depends on where the problem is. Cleaning the flame rod? You can do that yourself in about 20 minutes with a Scotch-Brite pad and a Phillips screwdriver. Checking the vent for blockages? Also DIY. But if your gas pressure is dropping under load, that's a gas company or licensed plumber call, no way around it. Don't mess with gas supply lines or regulators yourself. DIY repairs here cost you maybe $0 to $50 in parts. A tech runs $150 to $350 total depending on what's actually wrong and where you are.
How much does it cost to replace the flame rod on a Rinnai?
The part itself runs about $25 to $40 for a genuine Rinnai flame rod, part 100074985. If you're paying a tech, you're looking at maybe an hour of labor, so $125 to $200 total. It's honestly one of the easier repairs on these units. If you're comfortable with basic tools and following steps, this is a solid DIY job. Just shut the gas off at the unit before you start poking around in there.
Why does my Rinnai only throw Code 12 in winter?
A few things happen in winter all at once. Your furnace is running constantly and competing for gas at the same time as the water heater. Cold inlet water means the unit has to work harder and longer to reach setpoint, which stresses the gas supply more. Propane users especially see this because LP pressure actually drops in cold weather, sometimes enough to starve the heater. And vent terminations can get partially blocked by frost or ice buildup around the cap outside. I see way more Code 12 calls from October through February than the rest of the year combined.
How do I tell if it's the flame rod or the gas valve causing Code 12?
If you clean the flame rod and the code goes away for a few weeks then comes back, it's the rod slowly dying. If cleaning does nothing and the code comes back immediately, either the rod has a crack you can't see or the gas valve's modulating coil is failing. A cracked rod often tests fine with a multimeter when it's cold but still causes problems once it heats up. The gas valve is a lot more expensive, usually $200 to $400 just for the part, so always rule out the rod first before you go down that road.
Is Code 12 dangerous?
In normal operation, not really. When the flame drops out, the unit closes the gas valve within a couple seconds. But here's what you don't want to ignore: if you smell gas after the unit shuts down, that's a completely different situation. That would mean the gas valve isn't closing fully, and that's an emergency. Open windows, don't flip any switches, get everyone out, and call the gas company from outside. Code 12 by itself with no gas smell is annoying and inconvenient, but it's not dangerous.