Heat Pump - Low Pressure Fault - Heating

Article ID: 263
Last updated: 27 Feb, 2018

Heat Pump: YorkGuard VI

Re: Low Pressure Fault - Heating

These heat pumps are equipped with a YorkGuard VI module which monitors and controls unit operation and defrost.

A Low Pressure Fault is indicated by the following LED pattern on the outdoor unit control board:

  • Green = Off
  • Red = 4 Flashes

Low Pressure Switch

  • The heat pump is equipped with a low-pressure switch which is connected to the control at the LPS terminals.
  • If the low-pressure switch opens for more than five seconds, the control will cause a soft lockout condition and display the appropriate fault codes.
  • The control will ignore the low pressure switch input and not cause a soft lockout condition if it opens during the following conditions.
    • Defrost operation
    • First two minutes of compressor operation
    • Two minutes following the completion of a defrost cycle
    • TEST input shorted with Y1 input energized

► Unlike previous models, the YorkGuard VI does not flash the actual diagnostic code at the thermostat, only a code 2, 3 or 4 to indicate a "Soft" or "Hard" lockout.

Qty Possible Causes:
2 Bad Heating Expansion Valve
Low Refrigerant Charge
Bad Pressure Switch
Refrigerant Flow Restriction


1. Check Unit Operation

  • Make sure the thermostat is set correctly and calling for Heat.
  • Allow up to 5 minutes for compressor to start.
  • Remove the access panel of the outdoor unit.
  • Check the Diagnostic Indication Lights.
    • Unit still in Lockout ⇒ Remove 24VAC power to the board by removing the Red wire from the "R" terminal. Re-apply power after several seconds.

2. Check Refrigerant Pressures

  • Hook-up refrigerant gauges to the pressure ports on the unit.
    • Blue --> "True Suction" Low Pressure
    • Red --> High Pressure (either refrigerant line)
  • Ensure that the unit is running at full capacity (Red and Green LEDs both on).
  • Monitor pressures as the system operates.
Normal Pressure Range
Refrigerant 0-30 Deg O/D 30-50 Deg O/D
  • 75-100 Suction
  • 250-320 Discharge
  • 100-150 Suction
  • 250-375 Discharge
  • Normal Pressures + Low Pressure Lockout = Bad Pressure Switch

Low Pressure Issues

  • If a Liquid-Line Sight Glass is installed, observe for bubbles in the sight glass (after the system has been operating for at least 5 min.)
    • Low Suction + Low Discharge + Bubbles in Sight Glass + Expansion Valve Frosting = Low Refrigerant Charge
      • Try adding refrigerant through the Low Pressure gauge.
        • Suction and Discharge Pressures should gradually increase as refrigerant is added.
        • If the Discharge Pressure increases but the Suction stays low or drops = Bad Expansion Valve
    • Low Suction + Low Discharge + Clear Sight Glass + Expansion Valve Frosting = Bad Expansion Valve
This article was:   Helpful | Not helpful
Report an issue
Article ID: 263
Last updated: 27 Feb, 2018
Revision: 12
Access: Public
Views: 6794
Comments: 0