PM9 - Flashing Code 3

York 90% Gas Furnace: PM9

Re: Flashing Code 3

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

This gas furnace has one dual-port pressure switch:

• A pressure switch (PS1) monitors the differential pressure between the inducer and the sealed combustion box (typically around -.60" w.c.).

CHECKOUT:

1. Check Control Board

• Start the furnace with a call for heat.
• Ensure that power is present on "W" by checking for 24VAC between "W" and "C".
  • No Power = No Call for Heat
  • Power = 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 (with the inducer plug connected).
      • Power + No Inducer → Step 2. Check Inducer
      • No Power + Code 3 = Bad Transducer or Bad Control Board ⇒ Unplug wiring to the transducer and re-try.

2. Check Inducer

• Observe and listen to the inducer motor.
  • Inducer motor runs normally → Step 3. Check Pressure Switch
  • Inducer motor hums but does not start = Bad Inducer Motor or Item in Inducer ⇒ Remove inducer & check
  • Sound of Water "Slurshing" = Water in 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.
• Remove the burner box cover.
• Connect a "Tee" in the Pressure Switch hose from the Inducer Motor.
• Connect a pressure meter to monitor the operating pressure.
• Start the furnace with a call for heat.
  • Step 1 - When the inducer motor starts, it should ramp up to close the switch contacts.
    • Switch contacts do not close at rated pressure = Bad Pressure Switch
  • Step 2 - After the switch closes, the inducer motor should slow to open the switch contacts.
    • Switch contacts do not re-open = Bad Pressure Switch
  • Step 3 - After the switch opens, the inducer should ramp up to once again close the switch contacts.
  • Step 4 - After the switch contacts close for the 2nd time, the inducer should ramp to -1.00" w.c. and start an ignition sequence.
    • Inducer doesn't ramp to -1.00" w.c = Bad Transducer
  • Step 5 - After firing and proving flame, the inducer should ramp back down to "Minimum Fire" (and re-open the pressure switch).
• Re-Install the Burner Box Cover
  • Inducer Motor Speed drastically changes = Clogged or Restricted Intake Piping

4. Check Condensate Drainage

• Check both rain gutters on the inducer outlet.
  • Remove the drain hose and look into the coupling for any blockage.
    • Water runs out of the coupling = Clogged Drain Hose
  • Make sure the drain hoses are clear.
    • On horizontal runs, make sure the drain outlets are pointed slightly down-hill.
• Check the drain hose from the collector box and the inducer motor.
  • Make sure all drain hoses are clear and have a downward slope.
• Remove the drain trap and make sure it is not clogged.
• Make sure the drain line is clear.