4TWR4 - Flashing Code 4 - Heating

Article ID: 498
Last updated: 16 Dec, 2022

Trane Heat Pump: 4TWR3

Re: Flashing Code 4

Code 4 = 4 trips of the Low Pressure Control

  • 1st LPCO trip = 15 min. Soft lockout
  • 2nd LPCO trip = 30 min. Soft Lockout
  • 3rd LPCO trip = 4 hour (cooling), 18 hour (heating) Soft Lockout
  • 4th LPCO trip = Hard Lockout

Qty Possible Causes:
Low Refrigerant Charge
Bad Low Pressure Switch
Bad Fan Motor
Bad Expansion Valve
Defrost Issue


1. Reset & Check Operation

  • Reset the outdoor unit by cycling 24VAC power to the defrost board.
  • Make sure the compressor and outdoor fan motor restart

2. Check Refrigerant Pressures

  • Hook-up refrigerant gauges to the pressure ports on the unit.
    • In heating mode you should connect to the "True" suction and discharge ports behind the panel to the left of the control panel.
  • 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
  • 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.)
    • Bubbles = Low Refrigerant Charge
  • Low Pressure Issues
    • Low Suction Pressure + Low Discharge Pressure + 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 Pressure + Low Discharge Pressure + Clear Sight Glass + Expansion Valve Frosting = Bad Expansion Valve 

3. Test Defrost Cycle

  • Ensure that the Lite-Port LED is lit or flashing.
    • No Light = Bad Defrost Control or No 24VAC Power ⇒ Check for 24VAC between "R" and "B". (Unit will operate but not defrost without 24V power.)
  • Jump the TEST_COMMON to FRC_DFT pins to force the unit into defrost.
  • Defrost Sequence
  • Compressor & outdoor fan shut-off and the reversing valve energizes.
  • Compressor re-starts after approx. 20 seconds (outdoor fan stays off).
  • Unit stays in defrost until liquid line sensor reads 47 or 70 deg. (See Step 6. Defrost Termination Temperature)
  • Outdoor fan restarts for several seconds.
  • Compressor shuts-off & the reversing valve de-energizes.
  • Outdoor unit restarts after approx. 20 seconds.

4. Defrost Termination Temperature

  • The defrost control board has an adjustable termination temperature.
  • If everything tests correctly but the unit still has defrost issues, you can increase the defrost termination temperature.
  • As shipped from the factory, the termination temperature is 47 deg.
  • Clipping J2 (above the Test pins) changes the termination temperature to 70 deg. (Longer defrost cycle)
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Article ID: 498
Last updated: 16 Dec, 2022
Revision: 3
Views: 3267
Comments: 0