Performed diagnostic on 5yr old carrier heat pump, upon arrival customer stated unit was turning on and off every 20sec, also stated he had another company out a few months ago stating he need to replace damper, while talking unit started short cycling, checked tstats both in heat mode, upon checking outdoor unit can see frost after piston, also checked pressures and suction pressure was dropping down to 20psi causing low pressure switch to activate then once pressure goes back up to 40psi would turn back on and back off once it got to 20psi, checked reversing valve, made sure was shifting, checked temp across filter dryer 5 degree difference, explained system has a restriction, recommend removing all refrigerant cleaning piston and installing new filter dryer, customer approved repair, went to supply house to pick up parts, returned and removed all old refrigerant removed old filter dryer, pulled and cleaned piston, install new filter dryer, out piston back in, checked for leaks, pull