I have a Digitool, but for the most fun you want
Ken Lewis's wonderful little gizmo:
http://neksiwel.20m.com/photo5.html

Last winter he made some and almost gave them away.
I don't know how much he wants now or if he has
any, but there is an email link on this web page.

I installed one as he pictures and it is lots of
fun, poking along at Vanagon speed, watching the
lights dance back and forth--or typically, with it
pegged at max because the full throttle switch is
clicked.  You can tell if the O2 sensor is working,
the ECU is controlling it and if the idle and full
throttle switches are working properly.

                        Richard A Jones
                        Boulder, Colorado