Navien Tankless Water Heater Error Code 303: How to Fix It

Medium 45 minutes to 1 hour Medium Severity Verified June 2026
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Error Code
303
Brand
Navien
Product Type
water_heater
Severity
Medium
DIY Difficulty
Medium
Estimated Fix Time
45 minutes to 1 hour
Navien error code 303 indicates a fault with gas valve 1 on your tankless water heater. This typically points to a wiring issue, a stuck valve, or a failed component. Follow this guide to troubleshoot and resolve the problem safely.
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Tools You'll Need

How to Fix Error Code 303

  1. Turn Off Power and Gas

    Always shut off power and gas before servicing. Combustible gas can leak; do not create sparks.

  2. Inspect Wiring Connections

  3. Check Gas Valve Resistance

    Be careful not to short terminals. Disconnect power before measuring.

  4. Verify Gas Supply Pressure

    Gas pressure testing should be done with the system off. Consult a professional if uncertain.

  5. Replace Gas Valve

    Gas line work should be done by a licensed technician if you are not experienced. Ensure all connections are leak-tested with soapy water.

Parts You May Need

Navien Gas Valve (Gas Valve 1)
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Navien Gas Valve (Gas Valve 1)
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Navien Gas Valve (Gas Valve 1)
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Navien Gas Valve (Gas Valve 1)
Check Price on Amazon
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When to Call a Professional

If you are uncomfortable working with gas lines or electrical diagnostics, or if the error persists after replacing the gas valve, call a qualified Navien technician. Also contact a pro if gas pressure is abnormal or you smell gas.

Frequently Asked Questions

What does Navien error code 303 mean?
It indicates a fault with gas valve 1, meaning the valve is not operating correctly. Common causes include wiring issues, a stuck valve, or low gas pressure.
Can I still use my Navien water heater with error 303?
No, the unit will shut down and not heat water until the error is resolved. Using it with a faulty gas valve can be dangerous.
How do I reset Navien error 303?
Turn off the power and gas for 30 seconds, then turn them back on. If the error returns, further troubleshooting is needed.
Is gas valve 1 the same for all Navien models?
No, gas valve part numbers vary by model (e.g., NPE, NCB, NFC). Always check your owner's manual for the correct part or contact Navien support.
Can low gas pressure cause error 303?
Yes, low gas pressure can cause the valve to not open properly and trigger the fault. Check with a manometer.

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