Car wont pass inspection

Hello, my dad drives a Ford Crown Victoria cab which is also a natural gas car. He has been unable to pass inspection becasue the check engine light wont go away. He has had so many people look at it and have not been able to find the fix. This is how he makes his living so it has become a very stressful and serious situation. Mechanics think its something with the wires in the computers and nobody knows what exactly. Also since its a Nat gas car and everything is diferent, nobody can figure it out. Can you guys help out or recomend anybody who is an expert in this field especially in the NYC area. Especially for nat gas cars. Thanks so much.

Chris