2001 Ford Focus

Engine Mechanical problem
2001 Ford Focus 4 cyl Four Wheel Drive Automatic 158000 miles

I recently picked up my vehicle after having a new engine installed in the car, I drove 2 miles down the road the check engine light came on, I drove back to the repair shop the mechanic told me an old code was the problem, so he reset the light, thought things was fixed then 7 miles down th road the light came back on again, the car seems to be running fine, but I just don't understand if it is an old code prior to the old engine then why is the computer still picking it up. Just trying to understand. The mechanic told me to drive for a couple days see if it goes off, then to come back in if it doesnt. Could you explain why an old code may still be stuck in the computer.
May 25, 2008.

The computer run a check every time you start the engine
and if detect any malfunction in any of the monitors it will turn the check engine light on it warn you of it
its could be a simple vacuum leak to a major as converter no good or so on '
that why u need the codes
next time the light comes on have it scanned for codes
post it and we will let you know what is the issue

May 25, 2008.
Hi there,

when a code is deleted from the memory, the only way it will re set is if there is a new fault or an unresolved fault. The mechanic will have to scan and retrieve the code and rectify the system setting it before the light will stay off.

Mark (mhpautos)

May 25, 2008.