I had my 3.0L TDI A8 MOT'd a few weeks ago. It was hooked up to the emissions machine and nothing registered so another piece of equipment was plugged into the accessory socket inside the car, this is supposed to pick up a reading from the engine to prove it's being revved, so I can only assume it works on variable voltages as the revs go up and down. After a further 5 minutes of faffing around the tester told me he still couldn't get a reading and decided that all must be good and passed it...
Clean...dirty...who knows
