Performed preventative maintenance for York heat pump. I observed the system starting normally. The amp draw for the blower motor is within normal range. All electrical components are intact and operating properly. heat coils seem to be in good condition. Blower wheel is clean. Drain line is free of any clogs. The condenser turned on for a couple of minutes then the system defaulted to EM heat. The condenser has a blinking code for low pressure. I connected my gauges and noticed that the system in heat mode goes into negative pressure then shuts off. I ran the system in ac mode and found that the refrigerant pressures were within normal range. This tells me that there is an restriction with the reversing valve. The homeowner contacted his realtor and is contacting the company that installed the system. The system is only 2 years old.