Crank Shaft Position Sensor and check engine light

1. A friend/auto mechanic just replaced the timing belt and drive belts on the Accent. The engine sounds and drives fine and quiet.
2. After about 1hour of drive time, the check engine light lit up.
3. The scan at Advanced Auto said it was the crank shaft position sensor. Ordered the $40 one from Dorman Products. The one in stock at store was $113.
4. My husband replaced the sensor. I started the car and the check engine light went off. So I said, it was fixed and turned off the engine.
5. Later that day, I turned on the engine and the check engine light stayed on.

My next step is to return to an Advance Auto or Auto Zone to read the code. But. If I were an experienced mechanic, how would I trouble shoot this?

Since the sensor went as soon as the timing belt was replaced, is it coincidence?