Check Engine light on at high speed then broke the piston

[faiyolsystems]

I'm new in in this forum so correct me where I'm wrong.

 

My imprezza 2000 with ej15 engine had its engine opened because a very small problem which solved itself later. Later, the exhaust had a small dent and my mechanic argued it should be straightened and I agreed. These two modifications resulted in my car lighting the 'check engine light' whenever I'm driving at high speed 80+ km/h and goes off when I start driving at low speeds.

 

 

This behaviour recurred for like a month of going short distances of 20 to 30km per day. Last week but one, I had to go for a 250 km journey and that is when the problem blew out of proportion. 

 

the car started by generating very dark smoke from the exhaust and halfway through the journey,  the car broke down and could not move at all.

I had the engine opened a few days ago and one piston had broken down completely.

 

While I'm getting these broken parts replaced, I fear the mechanic does not know what caused this breakdown and this may recur in future . what I need to from you people is the following:

1. What would cause the piston to break down?

2. What is the likely cause of this 'check engine light' to be on after opening up the engine and the exhaust as I explained earlier?

kindly assist.

[lmdew]

How many miles on the car?

What codes were causing the CEL?

 

A dent in the exhaust wold not cause the piston problem.