MIL is on, code shows 456

Small leak in the evaporative system has been detected.

Evaporative system extends beyond the gas tank I am assuming.

Any help is appreciated.

Was MIL is on, possible causes.

The MIL or check engine light on 2002 Voyager 4 cylinder came on three weeks
ago. I took the car (mother in law's car) to local shop. They read the
code and it said the gas tank sensor was reading loss of pressure. I
bought her a new gas cap and cleared the reading. Three days ago, the
light came back on. My mechanic friend said he was not very familiar with
Voyagers (Chrysler).

Any clues beside taking it to dealer. Nearest dealer is 75 miles away.

Thanks