GMS8/GHS8/GDS8 - Short-Cycling, Won't Light - No Codes

Goodman Gas Furnace: GMS8/GDS8/GHS8

Re: Short-Cycling / Won't Ignite - No Codes

The furnace starts an ignition sequence, then shuts-off and re-starts at the point of ignition.

CHECKOUT:

1. Check Ignition Sequence

• Initiate a call for heat from the thermostat and monitor the ignition sequence.
  • The inducer motor should start with no difficulty.
  • The pressure switch should close within a couple seconds of the inducer starting.
  • The glow plug should energize within 30 seconds of the pressure switch closing.
    • No Glow from Glow Plug → Step 2. Check Glow Plug
  • The gas valve should power within 20 seconds of the glow plug energizing and stay open for at least 5 seconds.
    • Check for 24VAC across the two wires at the gas valve (while still connected).
      • No Power or Power Drops Quickly → Step 3. Check Control Board Output

2. Check Glow Plug

• Look for any visual cracks.
• Test resistance of the glow plug.
  • No Resistance = Bad Glow Plug
• Start an ignition sequence.
• Check for 120VAC power on the 2-wire plug feeding the glow plug during an ignition sequence.
  • No Power = Bad Ignition Control
  • Power Present + No Glow = Bad Glow Plug

3. Check Control Board Output

• Re-start the furnace with a call for heat from the thermostat.
• Ensure that the glow plug energizes.
• At the point of ignition, check for 24VAC between Terminal 9 (Blue) and Terminal 12 (Brown) of the molex plug on the control board. 
  • No Power = Bad Control Board or Shorted Gas Valve
  • Power at Board + No Power at Gas Valve = Tripped Auxilairy Limit (Mounted in blower compartment) ⇒ Check limit and blower operation.
  • No Power = Bad Control Board or Shorted Gas Valve → Step 4. Check Gas Valve

4. Check Gas Valve

• Remove the two wires from the gas valve.
• Re-start a call for heat and check for 24VAC power between the two removed wires.
  • No Power at Controller + No Power at Gas Valve = Bad Ignition Control
  • Power at Gas Valve ⇒ Temporarily plug the two wires into a new (uninstalled) gas valve and re-try an ignition sequence.
    • ​​​​​​​Gas Valve Powers for at least 5 seconds = Shorted Gas Valve 
    • No Power to Gas Valve = Bad Ignition Control Board