Status LED flashes four times, furnace shuts down shortly after ignition, may retry several times

Safety first: Turn off power before removing blower doors or reaching near moving parts.

Steps to check:

• Replace or clean air filter: Locate the filter rack (often in the return duct or blower compartment). Remove the filter and replace with the correct size and airflow rating recommended for your Gibson furnace. A severely clogged filter is the most common cause of limit trips.
• Check registers and grilles: Ensure all supply registers and return grilles are open and not blocked by furniture, rugs, or drapes.
• Inspect blower wheel and motor: Remove the blower compartment door. Check that the blower wheel is not packed with dust and spins freely. If heavily dirty, the wheel should be removed and cleaned. Verify the motor starts promptly when there is a call for heat.
• Examine high limit switch: The limit is usually mounted on the heat exchanger panel. Check for loose wiring connections. Do not bypass the limit switch; it is a critical safety device.
• If limit trips continue: Contact a technician to check static pressure, duct sizing, and heat exchanger condition.