Gas Furnace: PHU

Re: Flashing Code 3

If, after the vent blower is energized, the pressure switch does not close, the control will keep the blower on and wait for the switch to close. The diagnostic code for this problem is 3 short flashes followed by a pause.

► The diagnostic code will go away once the pressure switch closes.

CHECKOUT:

1. Check Control Board

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

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 or Item in Inducer ⇒ Remove inducer & check for obstruction
  • Inducer motor starts but is very noisy = Bad Inducer or Blower Wheel ⇒ Remove inducer & check wheel
• Ensure that the pressure switch closes.
  • Pressure switch fails to close → Step 3.  Check Pressure Switch

3. Check Pressure Switch

• Switch Contacts
• With the inducer running, check for 24VAC between each wire of the switch and "C".
  • Power on One Side Only = Switch Open
  • Power on Both Sides = Switch Closed
    • Switch Closed + Code 3 = Bad Control Board
• Switch Pressure
• Use a pressure meter to check the pressure at the switch.
• Compare actual pressure to pressure rating on the switch.
  • Pressure Near or Below Rating = Restricted Vent or Bad Inducer/Blower Wheel
  • Pressure Well Above Rating + Open Switch = Bad Switch