Furnace shows E10; inducer may start but burners do not light; may indicate reversed polarity or grounding issue

Safety first: This involves line voltage. If you are not trained in electrical work, contact a licensed electrician or HVAC technician.

• Verify power source: Turn off the breaker. Remove the furnace electrical access cover. Confirm that the hot (usually black) and neutral (usually white) wires are connected to the correct terminals or wire nuts as per the Napoleon wiring diagram.
• Check ground: Ensure the green or bare copper ground wire is securely attached to the furnace chassis ground screw.
• Test outlet or disconnect: If the furnace is cord-connected or fed from a nearby disconnect, use a plug-in polarity tester or multimeter to verify correct hot/neutral orientation and presence of ground.
• Correct wiring: If hot and neutral are reversed, correct the connections at the junction box or breaker panel as appropriate. Always keep power off while making changes.
• When to call a pro: If E10 persists after confirming correct polarity and ground, the control board's line-sensing circuit may be damaged and require replacement.