York 90% Gas Furnace: P3UR

Re: Flashing Code 3

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

CHECKOUT:

1. Check Control Board

• Start the furnace with a call for heat.
• Ensure that 24VAC is present between "W" and "C".
  • The Inducer motor should start within 15 seconds.
    • Inducer Motor Starts → Step 2. Check Inducer
    • Inducer Motor Does Not Start + Code 3 ⇒ Check for 115V power to inducer.
      • Power + No Inducer → Step 2. Check Inducer
      • No Power = Bad Control Board

2. Check Inducer

• Observe and listen to the inducer motor.
  • Inducer motor starts and runs normally → Step 3. Check Pressure Switch
  • 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 the vent piping.
  • If water is gurgling in the vent, look for low spots that will hold water.
    • The vent piping must have a constant slope back toward the furnace.

3. Check Pressure Switch

• Make sure there are no sags in the pressure switch hoses that could hold water.
• Switch Contacts
• With the inducer motor running, check for 24VAC between each side of the pressure switch and "C"
  • Alternative Method - Check for 24VAC at the molex plug between Pin# 7 -  White  and "C" and Pin #10 -  Blue  and "C".
    • Power on One Side Only = Switch Open
    • Power on Both Sides = Switch Closed
      • Switch Closed + Code 3 = Bad Control Board
• Switch Pressure
• Inspect the pressure switch tubing for a:
  • Loose hose
  • Dry-rotted, cracked hose
  • Clogged hose
• Use a pressure meter to check the pressure at the 1LP switch.
  • ► Must check positive & negative pressure together (differential pressure).
• Compare actual pressure to the pressure rating on the switch.
  • Pressure Near or Below the Switch Rating (0.90" w.c.) = Vent/Intake Issue ⇒ Check the negative pressure at the inducer and burner box individually.
    • Inducer Negative Pressure Low = Vent or Drain Problem or Inducer Problem ⇒ Remove inducer & check for obstructions or damaged wheel.
    • Burner Box Negative Pressure High = Intake Problem ⇒ Remove the burner box cover to see if pressure improves.
  • Pressure Well Above Rating + Open Switch = Bad Switch or Water in Switch

4. Check Condensate Drainage

• There are three drains that tie-in to the factory drain trap.
  • Vent Outlet
  • Inducer Motor
  • Collector Box
• Remove each drain hose individually.
  • Water running out of Drain = Clogged Drainage
• Remove the drain trap and flush-out at a sink.
• Check the main drain line.
  • In extreme cold conditions be cautious of Frozen Drains.