Check engine light

i was driving today and all of a sudden, my check engine light came on. I got a 98 xlt with automatic trans, v6 sohc with 145K miles. well, i went to a mechanic and he connected diagnostic tool and said the error was 455, something to do with fuel leak or emission. He said not to worry about it, can someone tell me what that error code is and what it means or what it can be.
Thanks

P0455 is gross emission leak. Usual suspect is bad sealing gas cap. Could be a hole in the vacuum system to the evaporative emission system somewhere.

P0455 is gross emission leak. Usual suspect is bad sealing gas cap. Could be a hole in the vacuum system to the evaporative emission system somewhere.

hopefully it was a bad sealed gas cap, but lets say it wasnt. what if there is a hole in the vacuum system, how does that affect the car? emission wise i dont care because my car is registered in montana where there are no emission testing. Is there a negative effect on the car itself?

Nothing of any big concern. May cause a small drivability problem due to vacuum leak at times. That should be about it.

I had the same problem last spring,..same error code P0455..and i had my mechanic take a look at it...and he said eveything seemed ok, but to bring it somewhere to have them hook up a machine that would pump smoke thru the system, therefore showing where/if there was a leak.

Before i did this, i took a leap of faith and went to the dealer.

Turned out there was a Field Service Bulliten on the issue and the Explorer i had (1997 sport) needed a software upgrade. The old software was telling the computer that there was an emissions issue when there really wasnt...and if i didnt get that upgrade, i wouldnt be able to pass inspection.

$80 later i had my upgrade and its been swell ever since.