It depends on the vehicle.  Our 2003 Matrix's OBDII tells us if we've left the gas tank cover loose!
 
James
Ottawa, ON
 
Date: Wed, 5 Mar 2014 07:26:01 -0600
From: ddbjorkman@VERIZON.NET
Subject: Re: Oil pressure warnings, WAS: Mann Oil Filter and lifters,              now emissions indicator lights.
To: vanagon@GERRY.VANAGON.COM

 Wait a minute Dennis.  My bus (87 Vanagon w/ VW Gen 5 inline gas) is OBDII and it doesn't tell me if the gas tank cover is loose :-)  > "maintenance indicator light" is there for. ..... These vehicles
> will even know if there is a fuel vapor leak such as a bad tank cap.
> 

> 
> Dennis