Navien NCB300-28K Service Manual page 63

Error type
Ignition failure
62 Troubleshooting
Cause
1. Test the primary and secondary voltages at the main gas valve.
Use a multimeter, between the YELLOW - WHITE wires and verify the voltage is
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DC 24 V
2. If there is no voltage, replace the PCB.
Main gas valve
3. If the voltage is correct, check if the coil is open.
Check the resistance. Check if the solenoid valve works correctly.
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Feel or listen for a click.
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Check that the coil is open (Correct resistance range: 100-1,000Ω)
Check the voltage at the solenoid valve / Yellow - White
Testing method