York 90% Gas Furnace

Re: Flashing Code 3

Flashing Code 3 = Furnace Pressure Switch failed to close on a call for heat.

Vent

All 90% gas furnaces have a pressure switch to monitor negative pressure from the inducer motor. If the venting system becomes clogged or restricted, the inducer motor will not be able to push enough exhaust and produce adequate negative pressure.

Intake

On 90% gas furnaces with a sealed combustion box, the pressure switch(es) will typically have a 2-hose connection - one hose to the inducer (-) and one to the burner box (+). This provides a differential pressure that protects against a restricted vent or intake pipe.

CHECKOUT:

1. Check Control Board

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

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
• Ensure that the pressure switch closes.
  • Pressure switch fails to close --> Step 3.  Check Pressure Switch
• Listen for sounds of water in the vent piping.
  • If water is gurgling in the vent, look for low spots that will hold water.
    • Pipe must have a constant slope back toward the furnace.

3. Check Pressure Switch

• Use a pressure meter to check the pressure at the switch.
  • Watch for water in the pressure switch hoses.
  • On a two-hose switch, be sure to check both sides at the same time (differential pressure)
• Compare actual pressure to the pressure rating on the switch.
  • Pressure near or below rating = Blocked or Restricted Vent / Intake
    • On a furnace with a sealed burner box, try removing the burner box cover & re-checking.
      • Drastic improvement = Restricted Intake
      • Little or no improvement = Restricted Vent
  • Pressure well above the rating, switch should be closed.
    • Switch does not close = Bad Switch or Water in Switch
      • Water in Switch is usually caused by a mis-run pressure switch hose.

4. Check Condensate Drainage

• Check the drain coupling on the inducer outlet.
  • On horizontal runs, make sure the drains outlets are pointed slightly down-hill.
  • Remove drain hoses.
    • Water running out of coupling = Clogged Drainage
• Check drain hoses from inducer and collector box.
  • Make sure all drain hoses are clear and have a downward slope.
• ! Make sure there is an "Air-Gap" in the PVC drain immediately upon exiting the furnace !
  • Any trap after the furnace before the air gap will cause drainage issues.
• In extreme cold conditions be cautious of Frozen Drains..