Bad bad service from Ford Warranty

I bought a 2015 Ford Fiesta, in 2016 but within the first service at ford the car already was giving me problems. I have serviced it with a different Ford dealership with 80000km, due to recurring overheating and cutting off problems. Finally took the car for service at Bosch for an oil change service and they diagnosed the problem of the overheating. Now I took the car back to Ford and gave them the report about the problem. Now am told Warranty cant cover due to the one service done outside Ford. Why was I to continue going back to Ford when I was never assisted for 80000km..