P3DH - Flashing Code 3

York Gas Furnace: TM9X/TM9E

Re: Flashing Code 3

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

• This gas furnace has two pressure switches (1LP & 2LP) - a higher pressure switch to monitor the negative pressure at the inducer motor, and a lower pressure switch to monitor for a clogged drain at the bottom of the collector box.
• Either switch open with the inducer motor operating will cause a Code 3.

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.
      • Power + No Inducer → Step 2. Check Inducer
      • No Power = Bad Control Board

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/Capacitor 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 by condensation over-running the rain gutter on the inducer outlet.
  • Listen for sounds of water in the vent piping (near the furnace and at the vent termination).
    • If water is gurgling in the vent, look for sags or low spots in the vent piping 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.
• Start the furnace with a call for heat.
• Check Drain Switch (Low Pressure Switch)
  • Switch Contacts
  • With the wires attached to the switch, check for 24VAC between each terminal of the switch and "C" Common.
    • 24VAC On Both Sides = Switch Closed ⇒ Check Vent Switch
    • 24VAC On One Side Only = Open Switch ⇒ Check Switch Pressure
  • Switch Pressure
  • Remove the Low Pressure Switch hose from the Collector Box.
    • Water Runs Out of Collector Box = Clogged Drain → Step 4. Check Condensate Drainage
  • Connect a pressure meter to the Collector Box pressure port.
  • Compare actual pressure to the pressure rating on the switch.
    • Pressure Near or Below Rating = Clogged Drain → Step 4. Check Condensate Drainage
      • Pressure Well Above the Rating + Open Switch = Bad Switch or Water in Switch
• ​​​​Check Vent Switch (High 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
  • Switch Pressure
  • Remove the High Pressure Switch hose from the Inducer Motor.
  • 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 the drain hoses from the collector box and inducer.
  • Make sure all drain hoses are clear and have a downward slope.
• Remove and clean-out the drain trap.
• Make sure the main drain line is clear.
• In extreme cold conditions be cautious of Frozen Drains.