A low pressure alarm indicates the controller has received an open signal from the normally closed low pressure switch circuit. Three active alarms within one hour will trigger a compressor lockout to protect the refrigerant circuit. The lockout can be cleared by power cycling the controller. The low pressure alarm is bypassed for the first 60s of compressor startup, which is adjustable through F1P in the L05 Menu if using the PLD or Cg in the Manufacturer Menu if using the PGD. 


Possible Cause
Component to Check
Recommended Action
Loose/incorrect alarm wiring
See Low Pressure Alarm Wiring below
If the low pressure switch is closed but the controller is receiving an open signal between ID3 and IDC1, verify/tighten alarm wiring according to the Low Pressure Alarm Circuit diagram.
Compressor start at extreme low temperature
Low pressure bypass
Low pressure bypass can be increased from default 60 seconds to 120 seconds through F1P in L05 Menu (PLD) or Cg in the Manufacturer Menu (PGD).
Lack of refrigerant
Low side pressure.
Normal Ranges:
R407C = 58-102 PSI
R410A = 115-175 PSI

1. Restart controller to clear lockout

2. Leak check unit

3. Repair leak if any

4. Charge appropriate amount of refrigerant

Low pressure switch defective
Low pressure switch
LP Switch Spec:
R407C: 18/36 ±5psi
R410A: 60/90 ±7psi
If switch is open when low side pressure is in normal range, replace the switch.
Reduced airflow
Supply fan
Air Filter
Verify that the supply fan runs properly and that the air filter is clean. If the fan does not run correctly, refer to A10/A11, Evaporator Fan Overload/AC Power Loss.

Low Pressure Alarm Circuit