Came out to call for a system not heating properly. Customer stated that the system was reading a code and the unit was blowing cold air non stop. I checked system over and found unit reading code 13, which states that the system had entered a lockout mode. This mode is activated due to the amount of short cycling a system does. The unit was being filtered at the unit and in the filter grill. Both filters were overdue to be changed. Though the customer stated the filters were changed a month ago. I recommended that the filter be changed every month, due to location of return and the amount of foot traffic and pets. I checked system operation, gas pressure and temperatures. System is heating properly within manufacturing specifications now.