Disconnect all modules from the class 2 bus, and probe the bus for shorts/opens.

Not having power/ground to all modules can cause disruption on the bus, as can a module that won't stop chatting on the bus (for some reason it is trying to complain about something), as can some shorts, as can pulling too much power.

To find an open in a power circuit:
- connect DMM - to battery negative;
- using DMM + probe power circuit, starting closest to battery:
--> if DMM shows exact battery voltage, then open is downstream (away from battery);
--> if DMM shows a slight drop, continue probing downstream;
--> if DMM shows a large drop or floating, then you have just passed the open.

To find an open in a ground circuit:
- connect DMM + to battery positive;
- using DMM - probe ground circuit, starting closest to battery:
--> if DMM shows exact battery voltage, then open is downstream (away from battery);
--> if DMM shows a slight drop, continue probing downstream;
--> if DMM shows a large drop or floating, then you have just passed the open.

Even if a connection looks good, probe either side of it...
Also spraying it with a small amount of WD40 will internally clean it up.


I don't like using the cigar lighter to power the scantool because doing this can set up a ground loop...

a ground loop is when to "grounds" which are at different potentials wrt to each other are connected... the current that flows between them is the difference between their potentials divided by the resistance of the connection, and often this current can be large and can fry something.