Navien Water Heater Error Code 031: How to Fix It

Medium 30-60 minutes Medium Severity Verified June 2026
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Error Code
031
Brand
Navien
Product Type
water_heater
Severity
Medium
DIY Difficulty
Medium
Estimated Fix Time
30-60 minutes
Error code 031 on your Navien tankless water heater indicates a short circuit in the flue gas thermistor. This sensor monitors exhaust temperature; a short disrupts safe operation, causing the heater to shut down. Follow these steps to diagnose and resolve the issue.
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Tools You'll Need

How to Fix Error Code 031

  1. Power Down the Water Heater

    Always disconnect power and shut off gas before servicing to prevent electric shock or gas leaks.

  2. Remove the Front Cover

  3. Locate the Flue Gas Thermistor

    Do not touch other internal components to avoid damage.

  4. Check the Wiring Connection

  5. Test the Thermistor Resistance

    Ensure wires are disconnected before testing to avoid false readings.

  6. Inspect the Wiring Harness

  7. Replace the Flue Gas Thermistor (if needed)

    Use only Navien-approved replacement parts to maintain warranty and safety.

  8. Reassemble and Test

    If the error reappears or you smell gas, immediately shut off gas and call a professional.

Parts You May Need

Flue gas thermistor
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Flue gas thermistor
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Flue gas thermistor
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Flue gas thermistor
Check Price on Amazon
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When to Call a Professional

Call a certified HVAC technician if you are uncomfortable working with gas appliances, the error persists after replacing the thermistor and checking all wiring, or you suspect a faulty control board.

Frequently Asked Questions

Is it safe to use the water heater with error 031?
No. Error 031 causes the heater to shut off for safety. Attempting to reset or use it without fixing could lead to dangerous overheating or exhaust issues. Fix the error before use.
Can a dirty sensor cause error 031?
A dirty sensor is unlikely; this error indicates a short circuit, not a cleaning issue. However, debris on the connector could cause a short, so cleaning the connection may help.
How much does a replacement flue gas thermistor cost?
A replacement thermistor typically costs between $20 and $40, depending on your model and supplier.
Can I bypass the flue gas thermistor?
No. Bypassing safety sensors is dangerous and voids your warranty. Always replace a faulty thermistor with a proper part.
Why does error 031 keep returning after I reset?
If the error returns, the thermistor likely has a permanent short. Replace it. Alternatively, there may be a wiring issue or a problem with the control board.

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