Lockout after excessive ignition retries (3 total)

What is going on

So the furnace tried to light three times and gave up, and now it's locked out. That just means the control never proved flame, so it shut the gas off for safety. Most of the time it comes down to something simple like a dirty flame sensor, but you want to check the gas and the igniter too.

Likely causes

• No gas getting to the burners, or a front cover pressure switch stuck open
• A bad or misaligned igniter, or the wrong orifices
• A flame sensor that is coated, oxidized or connected badly

Check in this order

1. Turn the power off before you start any repair.
2. Clean the flame sensor with steel wool, since a coated rod is the most common reason a flame drops out.
3. Handle the igniter gently, it is fragile, and check that it sits where it should.
4. Check the vent and flue piping against the vent section of the manual.
5. Confirm gas is reaching the burners. Gas pressure and orifice work goes to a licensed gas tech.
6. Reset the lockout and watch a full ignition cycle.

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)