I had the same problem after a fuel pump replacement. The code showed was a
loose or missing fuel cap so the dealer said that I did not have the fuel
cap on tight enough, which was BS. After 6 trips back to the dealer,
countless smoking of the fuel tank, a new fuel cap which did not solve the
problem, a new canister solenoid, a new purge valve assembly, the problem is
half arsed solved. I still occasionally get the CIL light on. It comes on
and then a day later goes out, with no code stored. I talked to a friend who
works for Esso and he said it might be the gas. Sometimes there is a bad
batch of gasoline with a slightly lower octane which will cause a misfire.
Go Figure.

After having a warranty replacement for the fuel pump on my Focus the
check engine light came on. I had the codes check and it indicated that
there was a problem with the gas cap. Could there be something else
causing the error code? Is there any type of conncection that might not
have been reattached after the fuel pump was replaced?