Fiat Charging Me Again and Again for the 2017 Fiat 500C

I own a 2017 Fiat 500C with 55,000 km on the odometer. In July 2022, I took it to an authorized Fiat dealer for a routine service, and there were no issues with the car prior to this. However, after the service, the "Engine Check" light began to illuminate, prompting me to return the car to the service center. Despite an extensive search, they couldn't identify the problem and concluded that we needed to rectify the engine, right after a timing belt change. I requested an engine replacement instead of a repair, and after a long wait, the new engine finally arrived in March 2023. I had hoped the issues would be resolved, but unfortunately, they persisted. The car still had the "Engine Check" light on, accompanied by excessive vibrations and knocking sounds. The service team informed me that the O2 sensors had failed, and I had to wait another three months for replacements. Even after the O2 sensors were replaced, the problems remained. They then claimed that the ECU was faulty and needed replacement, requesting additional payment. After waiting for about a month for the ECU change, the issues still persisted. Frustrated with the ongoing problems and additional charges, I reached out to Fiat Customer Relations and switched to another authorized service center. Now, they're telling me that the catalytic converter is damaged internally. Fiat refuses to cover this item and insists on charging me once again. It's clear that my only mistake was entrusting my car to an authorized dealer for a routine service. Fiat has refused to acknowledge any wrongdoing, even after a complete engine replacement and ongoing issues, continuously burdening me with additional expenses.