T957_O / A957_O: Low Refrigerant Pressure
On the VSODC, this fault will be indicated by the number 21 or L21. The L indicates that the alert has converted to a lockout.
The Drive LED's will not indicate anything
This fault is a low pressure event. If the event occurs within the same call 3 times, the system will switch from an Alert to an Alarm and lock the unit out for 1 hour.
Often this is caused by a system that is undercharged but it can also be caused by poor evaporator airflow, low load, or a restriction in the system. Use your skills to troubleshoot a low pressure event just like you would on any system.
It should be noted, that the low pressure is measured at the outdoor suction transducer. For (-)A20 models and heat pumps in cooling mode, the trip point for the low pressure switch is 50PSIG. The trip point for heat pumps in heating mode, the pressure triggers at 15 PSIG.
