Navien NCB-700-2S+/42K Service Manual page 71

Error type
Ignition failure
70 Troubleshooting
Cause
When no discharge is seen at the electrode during ignition:
Remove the electrode and check for cracks in the insulator.
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Adjust the gap if discharge is visible.
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Ensure that the insulating gasket is fitted between the electrode and burner casing.
No spark from the
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electrode
Check the input voltage at the ignition transformer (AC 230 V).
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If the voltage is correct, replace the ignition transformer.
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If there is no or low voltage at the ignition transformer, replace the PCB.
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Check the insulator boots on the spark leads for cracks or holes.
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Testing method
Ignition gap: 3.5-4.5 mm (1/8")
Ignition transformer