I got  a oil change and a coolant replacement done at the dealer and
drove home roughly 5 miles. In the evening ( afer 6 hours) when I
started the car the check engine light was on and the engine was
vibrating at the idle speed. Then I took the car back to the dealer and
they connected the diagnostic computer and found out that the oxygen
sensor is bad.
The mechanic  cleared the alarm and the engine started running normal.
They advised me to get the sensor replaced which costs around $300.
I am just wondering how the sensor could go bad right after the
service. Is there anything that I can do to find out whether the sensor
is really bad?

Thanks,
Ken