YP9C - Flashing Code 3

Article ID: 604
Last updated: 13 Mar, 2019

York 90% Gas Furnace: YP9C

Re: Flashing Code 3

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

►The code description on the blower door may say, "Vent or Termination Problem" which is a possible cause, but not the most likely.

This gas furnace has two pressure switches:

  • A higher pressure switch (PS1) that monitors the negative pressure from the inducer (typically around -.67" w.c.).
  • A low pressure switch (PS2) that monitors the collector box pressure for a clogged drain (typically -0.20" w.c.).

Only the PS1 switch will generate a Code 3. (PS2 is wired in series with the Roll-Out Switches and will generate a Code 5).

Qty Possible Causes:
Bad Inducer Motor
2 Bad Vent Pressure Switch
1 Water in Pressure Hose
1 Bad Pressure Transducer
Water in the vent (improper slope)
Item Stuck in Inducer Motor
Bad Control Board


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.
        • 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 Switches
    • Inducer motor hums but does not start = Bad Inducer Motor or 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
      • The only logical way for water to get into the inducer is through the vent (clogged vent drain).
  • 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 Switches

  • Make sure there are no sags in the pressure switch hoses that could hold water.
  • 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 -1.0" w.c. and start an ignition sequence.
    • Inducer doesn't ramp to -1.0" w.c = Bad Transducer
  • Step 5 - After firing and proving flame, the inducer should ramp back down to "Minimum Fire".

4. Check Condensate Drainage

  • Check the rain gutter 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 hose is clear.
      • On horizontal runs, make sure the drain outlets are pointed slightly down-hill.
  • Check the drain hose from the collector box.
    • Make sure all drain hoses are clear and have a downward slope.
  • IMPORTANT! 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.
    • A tee running up to the air conditioner coil will act as an air gap.
  • In extreme cold conditions be cautious of Frozen Drains.
This article was:   Helpful | Not helpful
Report an issue
Article ID: 604
Last updated: 13 Mar, 2019
Revision: 8
Access: Public
Views: 417
Comments: 0