。
She felt that something was off with her car.
Literal
She [topic-は] her own car [subject-が] somewhere condition [subject-が] bad [quotative-と] felt.
調子が悪い ('condition is bad, not in good shape') is the standard idiom for something running poorly — applies to machines (malfunctioning), people (feeling unwell), and projects (going off track). どこか ('somewhere') here functions as a hedge meaning 'somewhere or other' / 'in some way' — she couldn't pinpoint the issue but sensed it. Note the double が in 自分の車がどこか調子が悪い: the outer が marks the car as the topic of the embedded judgment, the inner が marks 調子 as the subject of 悪い.