My engine light went on last night?

I just had major work done last week, timing belt snapped caused value damage I had a new belt values, mechanic told me I have a new top half of my engine. I have a 12 month guarantee and I going to ask him tomorrow. I just wanted to know just for my curiosity why it would go on. I have 160, 000 miles on it 2005 Honda.

Dozens of possible reasons that may or may not be associated with the repair.

It's possible a wire of pipe has come loose after the engine was rebuilt, mechanic should fix that under his warranty.

Possible an engine sensor has failed after being reassembled. That's just bad luck

Something totally unrelated could have gone wrong, and again that's just bad luck too.

Need to wait for the mechanic to read the codes from the computer and see what it says.

Could be anything. It might not be related to the recent repair and it might be. Take it back and ask the mechanic.

If you question is "… Why it would go on." I have no idea what you mean.