GMP Gas Furnace - Kicking On and Off

Goodman Gas Furnace: GMP

Re: Furnace Cycles Off and On

The gas furnace occasionally kicks off and then re-fires.

CHECKOUT:

1. Check Diagnostic Indication

• Try to catch the furnace cycling-off & 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 4. Check Flame Sensor

2. Check Pressure Switch

• Monitor for 24VAC between the two "Yellow" wires attached to the pressure switch when the furnace shuts off.
  • No 24VAC = Switch "Closed" → Step 3. Check Limit Switch
  • 24VAC Present = Switch "Open"
• 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 Limit Switch

• Monitor for 24VAC between the two "Red" wires attached to the limit switch when the furnace shuts off.
  • No 24VAC = Switch "Closed" → Step 4. Check Flame Sensor
  • 24VAC Present = Switch "Open"
• Check the rated temperature rise on the furnace nameplate.
• Monitor the temperature rise as the furnace operates.
  • Temperature rise at or above rating and limit opens = Low Airflow or Over-Fired
  • Temperature below rating and limit opens = Bad Limit

4. Check Flame Sensor

• DO NOT CLEAN THE FLAME SENSOR BEFORE TESTING.
• 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!