GMS9 - Flashing Code 3

Goodman Gas Furnace: GMS9

Re: Flashing Code 3

If, after the vent blower is energized, the pressure switch does not close, the control will keep the blower on and wait for the switch to close.

The diagnostic code for this problem is 3 short flashes followed by a pause.

► There are 2 pressure switches in the furnace:  One connected to the Inducer, and one connected to the collector box.

► Only the Inducer Pressure Switch will cause a Code 3.  (The condensate switch breaks power to the gas valve and will cause a Code 1).

► The diagnostic code will go away once the pressure switch closes.

CHECKOUT:

1. Check Control Board

• Start the furnace with a call for heat.
  • Inducer motor should start within 15 seconds.
    • Inducer motor starts & runs normally → Step 3. Check Pressure Switch
    • Inducer motor does not attempt to start & shows Code 3 ⇒ Check for 115V power to inducer.
      • Power + No Inducer → Step 2.  Check Inducer Motor
      • No Power = Bad Control Board

2. Check Inducer Motor

• Start the furnace with a call for heat.
• Observe and listen to inducer motor operation.
  • Inducer motor hums but does not start:
    • Bad Inducer Motor or Capacitor
    • Item in Inducer ⇒ Remove inducer & check
  • Sound of water "slurshing" = Water in Inducer ⇒  Remove Inducer, pour out water, re-install inducer → Step 4. Check Condensate Drainage
• Listen for sounds of water in vent piping.
  • If water is gurgling in vent, look for low spots that will hold water.
    • Vent pipe must have a constant slope back toward the furnace.

3. Check "Inducer" Pressure Switch

• Switch Contacts
• With the wires attached to the switch, check for 24VAC between each terminal of the switch and Common.
  • 24VAC On Both Sides = Switch Closed 
  • 24VAC On One Side Only = Open Switch ⇒ Check Switch Pressure (below)
• Switch Pressure
• Remove the pressure switch hose from the Inducer Motor.
  • Watch for water in the pressure switch hoses.
• Connect a pressure meter to the Inducer Motor pressure port.
• Compare actual pressure to the pressure rating on the switch.
  • Pressure Near or Below Rating = Clogged or Restricted Vent
  • Pressure Well Above the Rating + Open Switch = Bad Switch or Water in Switch
    • ​​​​Water in Switch is usually caused by a mis-run pressure switch hose.

4. Check Condensate Drainage

• Check drain hoses from inducer and collector box.
  • Make sure all drain hoses are clear and have a downward slope.
• Remove and clean-out condensate trap.
  • In extreme cold weather, be cautious of frozen drains.