Gas Furnace

Re: Multiple Attempts to Fire

The gas furnace burners ignite and shut-off several times before eventually burning normally.

CHECKOUT:

1. Check Diagnostic Indication

• If it is possible, try to catch the burners cycling & check the diagnostic indication on the control board.
• Furnace Cycling Off and No Diagnostic = "W" Dropping Out, Low Flame Signal or Bad Control Board
  • Place a jumper between "R" and "W" and re-check.
    • Furnace Runs Normally = Thermostat Issue
    • Furnace Still Cycles & No Fault Indication = Low Flame Signal or Bad Control Board --> Step 3. Check Flame Sensor

2. Check Pressure Switch

• Monitor for 24VAC across the two wires attached to the pressure switch.
  • 24VAC Present = Switch "Open"
  • No 24VAC = Switch "Closed"
• Check the rated pressure of the switch.
• Place a pressure gauge in-line with the pressure switch hose.
• Monitor the pressure as the furnace operates.
  • Pressure at or near the rating and the switch opens = Restricted Vent or Inducer
  • Pressure well above the rating and the switch opens = Bad Pressure Switch

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 Control Board
  • MOVE SENSOR LEADS BACK TO "COM" and "V" BEFORE PROCEEDING!