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Open limit switch — overheating. Usually restricted airflow.
Brand: Trane (Also covers American Standard) | Equipment: Gas Furnace
4 LED flashes means the high-temperature limit switch has opened. The heat exchanger got too hot and the switch tripped to prevent damage.
What: Restricted airflow lets the heat exchanger overheat.
How to check: Inspect filter; replace if dirty or aged.
Fix: Install clean filter, run heat cycle.
Shop partsWhat: Too many closed vents creates static pressure spike.
How to check: Open every supply and return.
Fix: Open vents, clear returns.
What: Weak blower can't carry heat away.
How to check: Feel airflow at a register during heat cycle.
Fix: Check capacitor first, then replace motor.
Shop partsWhat: Switch fails in open position even at normal temps.
How to check: Tech measures resistance across switch when cold.
Fix: Replace switch.
Shop partsTry our interactive symptom diagnostic tool or browse Trane parts.