Burners ignite but shut off quickly; status LED shows 4 red flashes repeating; furnace may short-cycle on ignition attempts.

Safety first: Turn off power before working inside the furnace. Metal edges can be sharp; wear gloves.

Steps to check:

• Replace or clean air filter: Locate the filter in the return air cabinet or filter rack. If dirty or clogged, replace with the correct size and MERV rating recommended for your Coleman furnace. Never run without a filter.
• Check supply and return airflow: Ensure all supply registers and return grilles are open and not blocked by furniture, rugs, or drapes. Verify that duct dampers (if present) are open.
• Inspect blower operation: With panels in place and power restored, call for heat and observe through the blower compartment window (if present). Confirm the indoor blower starts after the burners ignite and runs at normal speed. If it hums, starts slowly, or stops, the blower motor or capacitor may be failing.
• Locate and test high limit switch: The high limit is usually mounted on the heat exchanger panel inside the burner compartment. With power off, check for continuity across the limit. If it is open and does not reset after the furnace cools, it may be defective and should be replaced with a Coleman-rated limit switch of the same temperature rating.

Persistent high limit trips indicate an airflow or heat exchanger problem. If replacing the filter and ensuring airflow does not resolve the issue, contact an HVAC technician to inspect the blower, ductwork, and heat exchanger.