Carrier furnace Code 34 — Ignition proving failure

What this error means

The burners tried to light but the control board never sensed flame. The overwhelmingly common cause is a dirty flame sensor (a $0 fix); others include the gas valve being off, low gas pressure, or a failing igniter. Three consecutive 34 failures escalate to Code 14 (ignition lockout), which auto-clears after 3 hours or with a 60-second power-off.

Symptoms

• Burners light briefly then shut off, or never light
• Repeated ignition attempts, then lockout
• Code 34 (or 14 after repeated failures) on the LED

Try this first (safe DIY steps)

1. Confirm the gas shutoff valve at the furnace is fully open
2. Power off 60 seconds to clear a Code 14 lockout, then restore
3. If it recurs: the flame sensor likely needs cleaning — see our universal dirty-flame-sensor guide (a 20-minute DIY with a screwdriver and emery cloth)
4. If comfortable, also check the burner area for dust/debris

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 cleaning the flame sensor doesn't hold, sensor replacement or igniter/gas-valve diagnosis runs $100–$250 for the sensor; service call $80–$250.