Dettson furnace attempts to start but shuts down with E5; inducer may run briefly, then stop.

Critical safety warning: An open rollout switch can indicate dangerous combustion problems, including carbon monoxide risk. If you see signs of flame rollout (scorch marks, melted wiring, burning smell), shut off gas and power immediately and contact a qualified technician.

• 1. Locate rollout switches: With power off, remove the burner compartment door. Identify the rollout switches near the burners or heat exchanger openings. Some are manual reset with a small red button.
• 2. Attempt reset (only if no visible damage): If there are no burn marks or melted components, press the reset button on the rollout switch. If it clicks, it was tripped. Do not reset more than once without finding the cause.
• 3. Inspect burners and flue visually: Look for heavy soot, rust flakes, or debris around burners and in the flue connection. Do not disassemble the heat exchanger yourself; this should be done by a technician.
• 4. Check venting: Ensure the exhaust vent is not blocked outside and that vent connections are intact.
• 5. If E5 returns: Do not continue to reset the switch. A cracked heat exchanger or serious venting issue may be present. Leave the furnace off and call a licensed HVAC technician for a full combustion and safety inspection.