Upon arrival system not operational. Control board showing error for internal control fault. Reset power to unit. Cycled several times. System is intermittently operational. Operational upon departure. Also noticed thermal stress throughout heat exchanger. Contactor is burnt and pitted. 21 yr old carrier g/e package unit on the roof. R-22 refrigerant. Recommend replacing hvac due to age/reliability, operating condition/cost of repairs, efficiency and r22 phaseout.