Trane furnace 2 flashes — External lockout (retries exceeded)

What this error means

The board gave up after repeated failed ignition attempts and locked out — Trane's legend calls it 'external lockout.' On light-commercial equipment Trane documents this as a 1-hour lockout that auto-clears. The classic root cause is a dirty flame sensor.

Symptoms

• Furnace attempts ignition several times then goes quiet
• Red status LED blinking in groups of 2
• Heat returns for a while after a power cycle, then locks out again

Try this first (safe DIY steps)

1. Power off at the furnace switch 30–60 seconds, restore (or wait out the 1-hour auto-reset)
2. If it locks out again: the flame sensor almost certainly needs cleaning — see our universal flame-sensor guide
3. Confirm the gas valve is on

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 a cleaned sensor doesn't hold: igniter or gas-valve diagnosis, $80–$250 service call plus parts.