Dettson furnace displays E15; unit may lock out after several failed ignition attempts.

Safety first: Turn off power and gas if you smell gas or see signs of overheating. Do not repeatedly reset a furnace that will not ignite.

• 1. Reset lockout: Turn off power to the furnace for 30 seconds, then restore. This clears the lockout so you can observe one ignition attempt.
• 2. Observe ignition: Watch the sequence: inducer starts, ignitor glows, gas valve clicks, burners light. Note at which step it fails.
• 3. Address root cause: Use the behavior and any accompanying codes (E2, E3, etc.) to guide repairs: clean flame sensor, inspect ignitor, verify gas supply, and clean burners.
• 4. Avoid repeated resets: If the furnace continues to lock out, leave it off and call a qualified technician. Repeated failed ignition can lead to raw gas accumulation.