Rinnai Code 79: Combustion Air Temperature Sensor Fault

Quick Answer

Rinnai Code 79 indicates a fault with the combustion air inlet temperature sensor. To fix this, inspect the sensor wiring for corrosion and replace the thermistor if it fails a resistance test.

Nine times out of ten when I show up for a Code 79, moisture got to the intake thermistor or the connector corroded. Ignore it and your water heater won't modulate right, so you'll get that annoying temperature roller-coaster in the shower. Check the sensor resistance first. The wiring harness is the second thing. Don't overthink this one.

RinnaiWaterheaterSeverity: moderateDifficulty: intermediate73% DIY Success

Time to Fix

20–60 min

Difficulty

intermediate

Parts Cost

$20 – $45

Tools Needed

Digital multimeter, Phillips #2 screwdriver

What Does the 79 Code Mean?

What catches most homeowners off guard is how sensitive this thermistor is to humidity and salt air. Nine times out of ten, we're looking at a simple resistance failure or a bit of green corrosion on the sensor plug. It's one of the more affordable repairs on a Rinnai, so don't jump to replacing the expensive control board until you've tested this $30 part first.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Combustion air inlet temperature sensor failure from age or corrosion40%

Wiring fault or corroded connector between sensor and PCB24%

Extremely cold combustion air inlet temperature beyond sensor design range14%

Moisture ingress into sensor element causing drift12%

PCB fault causing incorrect sensor signal interpretation10%

Symptoms You May Notice

Code 79 flashing on the controller display with the unit locked out and refusing to fire
The unit lights briefly, maybe even heats water for a minute, then cuts out mid-demand before you get consistent hot water at the tap
The fault shows up way more often in the dead of winter, especially if your intake pipe is pulling cold outdoor air directly from an exterior wall vent
No obvious physical signs from the outside, the unit looks completely normal but just won't stay running

Can you reset a Rinnai waterheater to clear the 79 code?

Once you have swapped the sensor or cleaned the terminals, power the unit down at the wall or breaker. Wait thirty seconds, then power it back on and cycle the hot water tap. If the code clears and the fan ramps up, you are good to go. If it returns immediately, the PCB might not be seeing the new sensor due to a harness break.

Tools Required for Diagnosis

Digital multimeterPhillips #2 screwdriverElectrical contact cleanerNeedle-nose pliers (for connector tabs)Flashlight or headlamp

Diagnostic Checklist

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

ComponentComponent Under Test

Expected Range10000–12000 ohms

ConditionIf Open (OL) or infinite, replace component.

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
Combustion Air Temperature Sensor (NTC Thermistor)Contact Rinnai with model number · $20–$45	Contact Rinnai with model number	$20 – $45

Frequently Asked Questions

What does the combustion air temperature sensor do on a Rinnai?

Think of this sensor as the unit's way of breathing correctly. It tells the computer exactly how cold the incoming air is so the fan can speed up or slow down to maintain the perfect fire. Cold air is way denser than warm air, so without this data, the water heater might run too lean or too rich, which leads to soot buildup and lousy efficiency over time.

How do I test a Rinnai temperature sensor with a multimeter?

You need to measure resistance across the two pins of the sensor while it's unplugged. At room temperature, look for about 10,000 to 12,000 ohms. If you hold the sensor in your hand, your body heat should make the resistance numbers drop steadily on your screen. That's the sensor working. If the numbers jump around wildly or the meter stays on OL, the sensor's toast and needs to be replaced.

Can Code 79 appear in cold weather without the sensor actually failing?

Yes, it happens more than you'd think. If your Rinnai pulls air directly from a vent on the side of the house, a polar vortex can push the sensor past its lower limit. I've seen units throw this code just because the intake air was too cold for the PCB to calculate a safe burn. Insulating the intake pipe or drawing air from a warmer crawlspace often clears the fault without needing any new parts.

How long does a Rinnai air temperature sensor last?

In a clean, dry basement, these sensors can easily last 15 years. But if your unit's in a laundry room with high humidity or near the coast where salt air is everywhere, the lifespan drops pretty fast. The tiny wires inside the thermistor are really sensitive to corrosion, which is why I always check the plug for signs of moisture during every annual service call.

Is Code 79 a DIY repair?

Honestly, this is one of the better beginner repairs on a tankless unit. The sensor's usually held in by a single screw or a rubber grommet, and the part's relatively cheap. As long as you're comfortable with a screwdriver and a basic multimeter, you can save yourself a $200 service call. Just be gentle with the plastic clips on the wiring harness so they don't snap.

Related Rinnai Waterheater Error Codes

Code: HUBRinnai Tankless Water Heater Error Codes: Complete Guide Code: 55Rinnai Code 55: Combustion Abnormality Detected Code: 32Rinnai Code 32: Outlet Water Temperature Too High Code: RESETHow to Reset a Rinnai Tankless Water Heater: All Models Guide

Same Fix on Other Brands

Navien waterheaterE773: Navien E773 Error Code: Exhaust Overheat Thermal Limit

Models Known to Experience 79 Errors

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

RUR98iN, RU199iN, RU98eN, V65iN, RL75iN, RU160iP

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 14, 2026