Flashing cel but engine doesn't seem to be misfiring?
 

Had my engine replaced 1500 miles ago and now all of a sudden as
I'm reaching the end Of the break in period im getting a flashing cel at about 4000rpm and above. It did it a few times and now it's stayed on. It doesn't feel like its misfiring though Which is confusing me. It runs perfectly otherwise. They didn't replace the coil packs as I had replaced them about 1000 miles before I replaced the engine. Anyone shine some light? Im taking it back to the mechanic on Monday anyway but I'd just like some help or knowledge! It's much appreciated.

Before you begin any troubleshooting, you are going to have to pull the stored code(s).