[Hanuman]

Reading a discussion on the subject for the V8 on a U.S. Site, it transpired that on North American market cars, (whether all, or just certain m.y chassis, I don't know), the check engine light bulb was omitted, but otherwise all of the circuitry was present.
For commonality, there was little mention of its function, but someone found out by trial and error, that fitting the bulb in the dash permanently allowed the blink codes to display on the dash, instead of messing about hanging upside in the footwell prodding bits of random wire in holes.

At least, that's how I understood it. (There was still some prodding, but not so much) I had to read between the lines of americanglish, so some of it might have been lost in translation.

To John Clayton, I should say-
One point of caution in your original post on this topic, is the 'accessing the diagnostic program.'

It's not quite that advanced. If your garage are expecting some wizard output that says 'code/sensor FUBAR', they might be sorely disappointed.

What actually comes out is a four character blink code. That is then referenced to a table by the very human part of the diagnostic program, ie. the Technician, or the Technicians Monkey, which will give a very broad reference to a sensor not giving a correct output.
It's fun, but archaic.

There's some notes about this (MAC05) on Phil Paynes Isham research website.


Two notes-
Autocorrect has a slightly Freudian aspect to its function, me thinks;
FUBAR autocorrects to 'Dubai', and Isham to Islam.
Neither of which have any practical value in this discussion, btw.