You finally got the repair done. The shop replaced the sensor, fixed the leak, or installed the new part you were told would solve the problem, and the check engine light went away. But then, days or weeks later, it came back.
That can be frustrating, especially when you've already paid for the repair. But a returning check engine light doesn't necessarily mean the mechanic replaced the wrong part. In many cases, the original repair addressed one symptom while another part of the same system is still causing trouble.
