Furnace shows E12; blower may start but burners do not ignite; often associated with limit circuit issues

Safety first: Turn off power before checking limit switches or wiring.

• Trace limit circuit: Using the Napoleon wiring diagram, identify all limit switches in series (high limit, auxiliary limits, rollout switches). Check each for secure wiring.
• Continuity test: With power off, disconnect one wire at a time and test each limit switch for continuity at room temperature. Any switch that is open must be investigated.
• Inspect for overheating: If a limit is open and shows signs of heat damage, determine why the furnace overheated (blocked filter, closed vents, blower failure) before simply replacing the switch.
• Repair wiring: Repair or replace any broken or burned wires in the limit circuit using high-temperature-rated wire.
• When to replace: Replace any failed limit switches with Napoleon-approved parts. If all switches and wiring are good but E12 remains, the control board limit input may be defective.