Trane furnace 4 flashes — High-temperature limit open

What this error means

The furnace overheated and the limit device opened. Nearly always restricted airflow: clogged filter, blocked returns, closed registers, or a dirty blower/coil.

Symptoms

• Furnace short-cycles: heats briefly, shuts down, repeats
• 4-flash pattern on the LED
• Weak airflow at the vents

Try this first (safe DIY steps)

1. Replace the filter (the right spec — overly restrictive high-MERV filters trigger exactly this)
2. Open all registers and clear the return grilles
3. Power off 30–60 seconds and restore

Safety: power off before any physical intervention. Never bypass covers or interlocks — fuser areas run hot enough to burn.

When to call a technician

If it persists with a fresh filter: blower or coil cleaning/diagnosis; limit switch replacement runs $150–$400 if the switch itself has failed.