HELP!! HUGE MYSTERY on this Chevy cavalier!!! Any ideas about why the check engine light is ALWAYS on

Question

So there was an engine in before and it was replaced, before it blew up and had to be replaced the check engine light was always on, now, there is a used engine in and a brand new transmission, the check engine light was off and everything was fine until today while it was being driven, the check engine light came on for about 5 minutes then went off, then came on again for about 20 minutes, etc etc...it went off and on while driving basically, the only thing that is known for sure is that the transmsion and engine were BOTH repalced recently, and the transmssion is brand new, but the engine is used and it did fail smog because of an 02 sensor? What do you think could be wrong, and it was doing the exact same thing before when the engine BLEW UP completely? Don't want to ruin a second engine, it is always driven gingerly, and oil changes are always done on time, why in the world is it doing the exact same thing again with the REPLACEMENT engine? and BRAND NEW transmission? The reason to ask here and not shops is that the shop workers are pretty lazy they just say they don't know and charge credit card for the inspection, and don't even do preventive maintenance. It is a year 2001 Chevy Cavalier

Answer 1

It's a Cavalier there is always something wrong with them. However go get an OBD-II code reader from autozone and find out what your ECU says is wrong.

Answer 2

You need to go to a local auto parts store and ask them to check the OBD2 codes.  This will give you a read out of the problem. Most of the time it is an 02 or some other sensor. Cam sensors go out frequently also. This should be free to diagnose.