Rinnai Code 79: Combustion Air Temperature Sensor Fault

Quick Answer

Rinnai Code 79 indicates a fault with the combustion air inlet temperature sensor. This sensor monitors the temperature of the air entering the combustion chamber and feeds data to the PCB to optimize the combustion process. A failed sensor or one reading outside its calibrated range triggers Code 79. Sensor failure from age or corrosion is the most common cause. Wiring faults between the sensor and PCB are the second most frequent cause. In rare cases, the installation environment may be producing air temperatures far outside the sensor's design range, such as extreme cold air entering from an uninsulated exterior air inlet in very cold climates. Sensor replacement is typically a simple repair.

RinnaiWaterheaterSeverity: moderateDifficulty: intermediate73% DIY Success

Time to Fix

20–60 min

Difficulty

intermediate

Parts Cost

$20 – $45

Tools Needed

Multimeter, Screwdriver set

Wrong code?

Not seeing 79 on your display?

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

combustion temperature sensor fault

NavienE773

What Does the 79 Code Mean?

Combustion air inlet temperature sensor is failing or reading out of range. Usually a sensor replacement or wiring issue. Less common than ignition or fan codes.

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 on controller display
Unit refuses to fire or shuts down shortly after demand
Code appears more frequently in winter on cold air inlet installations
No physical symptoms visible externally

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

After replacing the sensor or repairing wiring, restore power at the breaker and press the power button off and on to clear Code 79. The unit should proceed through its startup sequence normally. If Code 79 returns on the first heating demand, verify the sensor connector is fully seated.

Tools Required for Diagnosis

MultimeterScrewdriver setElectrical contact cleanerNeedle-nose pliers (for connector removal)

Diagnostic Checklist

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

Note the current ambient air temperature at the unit's air inlet

Note the current ambient air temperature at the unit's air inlet. Code 79 on a very cold winter day with an outside-air inlet may simply mean the air temperature is at the edge of the sensor's rated operating range. A single occurrence under unusual temperature conditions may be transient.

Cut power at the breaker

Set your multimeter to resistance (ohms) mode

Inspect the wiring harness from the sensor to the PCB

If the sensor tests good

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

Verify part fitment for your Rinnai model

Repair vs. Replace Calculator

Appliance Age (Years)7 Yrs

Labor Approach

Our Verdict

REPAIR

Estimated Cost: $33 (Parts) vs $1250 (New Machine)

Frequently Asked Questions

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

The combustion air inlet temperature sensor measures the temperature of the air entering the burner. The PCB uses this data to adjust the air-fuel ratio and burner modulation for optimal combustion efficiency and emissions control. Cold inlet air is denser and affects the combustion equation differently than warm air, so the sensor input is used to compensate automatically.

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

Disconnect the sensor from its wiring harness. Set your multimeter to the ohms (resistance) setting. Touch one probe to each sensor terminal. At room temperature (approximately 68F / 20C), an NTC thermistor used for air temperature sensing in Rinnai units typically reads 10,000 to 12,000 ohms. Readings of OL (open circuit) or near zero ohms confirm sensor failure.

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

Yes, in rare cases. If the unit draws combustion air from an uninsulated exterior duct in a very cold climate, the inlet air temperature can fall below the sensor's lower design range. This can cause the PCB to treat the sensor reading as an error. Adding insulation to the exterior air supply duct or routing the air inlet from a conditioned space can address this specific installation issue.

How long does a Rinnai air temperature sensor last?

NTC thermistor sensors typically last 10 to 15 years under normal conditions. Exposure to moisture, corrosive combustion air (near salt water, or in coastal environments), or high humidity accelerates degradation. Annual inspections during descaling maintenance are a good opportunity to check sensor connectors for corrosion.

Is Code 79 a DIY repair?

Sensor replacement is a reasonable DIY repair for someone comfortable with basic multimeter use and working inside an appliance with the power off. The sensor itself is a low-voltage, low-cost component. The main risk is damaging the wiring harness during removal. If the code persists after sensor replacement, further diagnosis involving the PCB should be left to a technician.

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

Last verified for technical accuracy on March 14, 2026