PHU - Flashing Code 3

Article ID: 345
Last updated: 16 Nov, 2021

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.

Qty Possible Causes:
4 Bad Inducer Motor/Wheel
1 Bad Pressure Switch
1 Item Stuck in Inducer
Bad Ignition Control


1. Check 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.

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 

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Article ID: 345
Last updated: 16 Nov, 2021
Revision: 16
Access: Public
Views: 65
Comments: 0