Code: 51🚿 Noritz Tankless Water Heater

Noritz Error Code 51: Gas Solenoid Valve Failure (Meaning, Causes, Fixes)

Reviewed 2026-10-24 · Based on 2 manufacturer & service sources

What the code means

Noritz service manuals list error code 51 as Failure of Gas Solenoid Valve (or Gas Solenoid Valve drive circuit abnormality). The display window on the unit may show 51 or 511, while the remote controller shows 51. The gas solenoid valves open and close to let gas reach the burner: the primary valve (SV0) sits on the gas valve and three more (SV1, SV2 and SV3) sit on the manifold plate. Noritz says the code appears when a gas solenoid valve's connections are improper, when those connections are damaged, or when the circuit board has failed. The error table says that to reset code 51, power has to be disconnected and then reconnected. The diagnostic flow chart has a technician check whether any of the SV0 to SV3 connectors on the manifold plate and gas valve came loose, then measure each valve's coil resistance and inspect the wiring. A bad SV0 means replacing the gas valve, a bad SV1, SV2 or SV3 means replacing the manifold plate, and if the valves and wiring test normal, the circuit board is replaced. Noritz lists a separate code, 71, for a gas solenoid valve drive circuit abnormality caused by a circuit board failure.

Likely causes

Safe DIY checks

⚠ Before you start

Unplug the appliance and shut off its gas valve before working on it; never disconnect or alter the gas line. If you smell gas, leave and call your gas utility.

  1. Turn the heater's power off and back on. Noritz's error table says that is how code 51 is reset.
  2. If 51 comes right back, stop resetting it and call for service. The fix involves the gas valve, manifold plate or circuit board inside the unit.
  3. Don't open the gas valve or manifold yourself. Noritz lists 51 in its technician service documents, not in the homeowner troubleshooting steps.
  4. Write down your model number from the rating label and whether the display shows 51 or 511, so the technician can bring the right parts.
  5. If you ever smell gas, leave the area and call your gas utility or a qualified technician right away.

🛠 When to call a pro

  • Checking whether any of the SV0 to SV3 connectors on the manifold plate and gas valve have come loose, and reconnecting them.
  • Measuring the coil resistance of each gas solenoid valve with the connector unplugged, per the Noritz diagnostic flow chart.
  • Replacing the gas valve if SV0 is faulty, or the manifold plate if SV1, SV2 or SV3 is faulty.
  • Replacing the circuit board if the valves and wiring test normal (Noritz says the display window board does not need replacing).

Parts that commonly fix it

Always match the part number to your exact model.

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Digital multimeter

Used by technicians to check gas solenoid valve coil resistance against the Noritz service manual.

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Gas leak detector

Handy to have near any gas appliance, especially after gas valve work.

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Carbon monoxide alarm

Keep a working CO alarm near any gas water heater.

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Sources

  1. Noritz EZ series Service Manual: Error Codes and Checkpoints p.31 (51/511 Failure of Gas Solenoid Valve or drive circuit abnormality; improper or damaged valve connections, circuit board failure; reset by disconnecting and reconnecting power) and Error Code Diagnostic Flow Chart p.49 (SV0 to SV3 connectors, coil resistance, replace gas valve, manifold plate or circuit board) (HVAC Market copy)
  2. Noritz NR501-OD Service Manual p.45: Failure of Gas Solenoid Valve (51, 511), same conditions and diagnostic flow (ManualsLib)

Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.