Code 21 — Furnace Gas Heat Lockout

What does 21 mean?

This fault indicates that the furnace has gone into a lockout mode specifically for its gas heating function. This usually occurs after multiple failed attempts to ignite or sustain a flame.

• No gas supply or low gas pressure
• Ignition system failure (igniter or flame sensor)
• Blocked flue or exhaust
• Faulty gas valve

Click steps to track your progress.

1. 1

   Verify that the gas valve to the furnace is open and other gas appliances are working.

2. 2

   Turn off the power to the furnace at the circuit breaker for at least 30 seconds to reset.

3. 3

   Ensure that the flue and intake vents for the furnace are clear of obstructions.

4. 4

   If the lockout persists, contact a qualified HVAC technician to diagnose the gas heating system.