Furnace attempts ignition but shuts down; LED flashes four times repeatedly; blower may run for a short purge cycle.

Safety first: Turn off power before removing access panels. Components near the burners and heat exchanger can be hot.

• Replace or clean air filter: Locate the return air filter (often in the blower compartment or return grille). If dirty or clogged, replace with the correct size and airflow rating recommended for your Fraser‑Johnston furnace.
• Check supply and return registers: Ensure that at least 80% of registers are open and not blocked by furniture or rugs. Closed registers can cause overheating.
• Inspect blower operation: With panels in place and power restored, call for heat and observe the blower start. If the blower is slow to start, noisy, or not running at all, the blower motor or capacitor may be failing.
• Check high limit switch wiring: With power off, locate the high limit switch mounted on the heat exchanger panel. Verify that the wires are firmly connected and not burned or loose.
• Reset lockout: After correcting airflow issues, turn power off for 30 seconds to reset the control. If the 4‑flash code returns frequently, the high limit switch or blower system should be tested and possibly replaced by a technician.