1998 Volvo S70 check engine light.

1998 Volvo S70 Front Wheel Drive 115000 miles

The car has a code 455 large leak fuel tank.
I took it to the dealer, Royal Motor of san francisco.
They did the smoke test, the found no leak at this time
it cost $180 sep 07.
The light still on code 455, I took it to McKevitt volvo, san leandro. Ecm-611 large leak permanent fault stored. Clear fault code and perform leak test vehicle passed. System may have intermitent fault or gas cap was left loose. This is exactly how is written in the work order. Gas cap has been replaced several times, it was jan 09.
Still check engine light is on, what can I do.