Gas Furnace - Gas Fires then Shuts Off

Gas Furnace

Re: Burners Fire then Shut Off

The furnace goes through a normal ignition sequence, but several seconds after ignition the burners shut off.

CHECKOUT:

1. Check Power to Ignition Control

• Check for 24VAC power between "TH" or "24V" on ignition control and "C" on the terminal block.
• Monitor power through the ignition sequence.
  • Power drops out when burners shut off = Pressure Switch Opening or Thermostat Short-Cycling
  • Power stays on but burners shut off → Step 2. Check Ignition

2. Check Ignition

• Visually inspect the burners during ignition.
  • Burners should light with little or no delay from the glow plug to the flame sensor.
    • Make sure flame sensor comes in good contact with the flame.
    • Any delay or burners not igniting = Clogged Burners
    • Burners light properly →  Step 3. Check Flame Sensor

3. Check Flame Sensor

• On the multi-meter:
  • Switch the test leads to "COM" and "mA".
  • Set the meter on DC 200ù.
  • Disconnect the wire from the flame sensor.
  • Place one meter lead on the flame sensor terminal and the other on the flame sensor wire.
  • Check the reading when the burners ignite.
    • Minimum acceptable reading = 1 MicroAmp
      • Low Reading ⇒ Clean Flame Sensor & Re-check
      • Normal Reading (2 to 6 mA) but Shuts Off = Bad Ignition Control
  • MOVE SENSOR LEADS BACK TO "COM" and "V" BEFORE PROCEEDING!