Rinnai Code 12: Flame Failure During Operation

Quick Answer

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. Low flow rate causing the unit to cycle off mid-heat and vent restriction creating combustion backpressure are also common triggers. Check that the gas meter and regulator can supply adequate pressure under full load. Reset after addressing the root cause.

RinnaiWaterheaterSeverity: highDifficulty: intermediate72% DIY Success

Time to Fix

20–120 min

Difficulty

intermediate

Parts Cost

$12 – $80

Tools Needed

Screwdriver set, Fine steel wool or emery cloth

Wrong code?

Not seeing 12 on your display?

Select what your appliance is actually doing and we'll take you to the right guide.

flame goes out mid-cycle

NavienE012

What Does the 12 Code Mean?

Flame established during ignition but went out during operation. Gas pressure drop mid-cycle and a deteriorating flame rod are the top two causes, and they behave differently from Code 11.

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%

Flow rate dropping below 0.5 GPM minimum mid-cycle (pressure fluctuations in supply)14%

Gas valve not maintaining steady fuel delivery under modulated load12%

Vent restriction creating combustion backpressure10%

Symptoms You May Notice

Code 12 on controller display
Hot water starts normally then goes cold mid-shower
Unit shuts down after 30 to 90 seconds of operation
Code appears more often when multiple gas appliances run simultaneously
Occasional Code 11 on the same unit (indicates advanced flame rod wear)

Can you reset a Rinnai waterheater to clear the 12 code?

After addressing the identified root cause, press the power button to turn the unit off and on again. Allow 2 minutes before running hot water. If Code 12 reappears within the first heating cycle, the root cause has not been fully resolved. For gas pressure issues, a reset alone will not fix the problem.

Tools Required for Diagnosis

Screwdriver setFine steel wool or emery clothManometer (for gas pressure testing)Flow meter or bucket and timer (to verify 0.5 GPM minimum)Flashlight

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

Determine whether Code 12 occurs consistently or only during

Determine whether Code 12 occurs consistently or only during specific conditions. Run hot water alone with all other gas appliances off. Note whether the code still appears.

Check incoming gas pressure if you have access to a manometer

Remove the burner access panel and inspect the flame rod

Inspect the vent pipe run from the unit to the exterior

Verify that the flow rate through the unit stays above 0.5 GPM

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

Verify part fitment for your Rinnai model

Repair vs. Replace Calculator

Appliance Age (Years)7 Yrs

Labor Approach

Our Verdict

REPAIR

Estimated Cost: $46 (Parts) vs $1250 (New Machine)

Frequently Asked Questions

What is the difference between Rinnai Code 11 and Code 12?

Code 11 means the unit never successfully ignited during 3 attempts. Code 12 means the burner lit but the flame dropped out while heating was in progress. The distinction matters for diagnosis: Code 11 points to ignition system components (flame rod oxide, igniter gap, gas at startup), while Code 12 points toward sustained operation issues like gas pressure drops, flow rate dips, or vent backpressure.

Why does my Rinnai only show Code 12 in the morning?

Morning is typically peak gas demand in a neighborhood when many people shower, cook breakfast, and heat their homes simultaneously. The increased load on the gas distribution network can cause momentary pressure drops that fall below your unit's minimum operating threshold. If Code 12 only appears during morning peak hours, contact your gas utility to verify street pressure and check whether your meter or regulator is undersized.

Can low water pressure cause Rinnai Code 12?

Indirectly, yes. Rinnai requires a minimum 0.5 GPM flow rate to sustain operation. If your household water pressure fluctuates during a shower (for example, a toilet flushes and diverts cold water), the flow rate through the water heater can momentarily drop below the threshold, causing the unit to shut off and log Code 12. Install a pressure-balancing shower valve to eliminate this pattern.

My Rinnai shows Code 12 only in winter. Is that normal?

Cold weather introduces two Code 12 triggers. First, LP tank pressure drops significantly when the tank is below 20% capacity and temperatures are low. Second, wind can cause backpressure at the vent termination cap on certain wall penetrations, disrupting combustion under sustained operation. Check LP level and consider adding a vent termination wind guard if cold-weather Code 12 is a recurring issue.

How long does a Rinnai flame rod last?

A flame rod typically lasts 5 to 10 years in normal operation. In hard water areas or where the unit cycles very frequently (such as a busy household), the rod may degrade faster. Annual cleaning during a maintenance visit can extend service life significantly. Once the rod tip is visibly eroded or the ceramic shows a crack, replacement is the correct action.

Related Rinnai Waterheater Error Codes

Code: HUBRinnai Tankless Water Heater Error Codes: Complete Guide Code: 11Rinnai Code 11: No Ignition Fix for RU and V Series Tankless Heaters Code: 10Rinnai Code 10: No Gas Supply or Gas Valve Failure Code: RESETHow to Reset a Rinnai Tankless Water Heater: All Models Guide

Same Fix on Other Brands

Navien waterheaterE012: Navien E012 Error Code: Flame Failure During Operation

Models Known to Experience 12 Errors

This repair applies to most Rinnai waterheaters with this error code. Common model numbers include:

RUR98iN, RU199iN, RU98eN, V65iN, RL75iN, RU160iP

Last verified for technical accuracy on March 14, 2026