OBD1 OBD2 Diagnostics
So your MIL/Check Engine/Service Engine light is on and you want to know why.
On the later vans that have OBD2(OBDii) you will need some type of scanner. I like to use my smartphone or tablet for doing my diagnostics using a bluetooth adapter and CarGaugePro for Android. Many LAPS will scan the codes for you for free, but for less than $30 total I like to keep this in my glove box.
Car Gauge Pro (OBD2 + Enhance)
NOTE: I DO NOT RECOMMEND THEM AS THEY ABANDONED THE OLD APP CREATING A "NEW" APP, forcing people to buy again for the "new" app. Shady
On earlier vans you can use a jumper, like a paperclip, on the DLC and the MIL will flash codes at you letting you know why the computer is lighting the MIL.