First...
Welcome to the forums
Your not saying what year, but your profile shows 1996 -- which means you're fully OBDII compliant.
You need to get yourself an OBDII code reader, plug it into the port just above the gas pedal, under the dash.
Looks like this one:
Let it read the code(s), most scanners can also reset the CEL (check engine light) if that's what your after.
The middle of the line Scan Tool looks like this one: About $100 USA
After you get the code(s) from the system, you can choose to clear them to see if they return right away -- If so, use a diagnosis table such as
THIS ONE to give you an idea of whats causing the CEL to come on.
If you can't find a proper diagnosis for the code(s) you get off your Limo, you can always come back here and post your code numbers -- Lot's of OBDII guys here.
BTW:
There's another Limo man on your side of the lake (I'm in California) that moderates
over here. Pop in there and say hi to Dereck....