Service contracts run out without anyone noticing. Held with term, due date and contact, they become recurring work instead of a list of old customers.
Every installation has a history: model, refrigerant or fuel, year, last service, parts fitted. Held by address, the engineer knows before leaving what is waiting and what to take.
In the busy months it is the spare part that fixes the date, not the diary. Order, delivery date and affected job together show which customer really gets seen this week.