engine carbon clean is a giant scam thats why you have to have it repeated every few months, the ignorance of the idiots doing it is astounding though as they all want a fast buck. You connect the machine to the intake and it pumps oxygen and hydrogen in, the hydrogen acts as a fuel and over the 30 mins it is connected the extra fuel is recognised by the o2 sensors which re-learn their fuel tables and make your engines management system inject less fuel so when you remove their device your emissions appear lower as the engine is running lean and not adding enough fuel to compensate until it re-learns its parameters, this gives the engine the feeling of more power as its not bogged down with an over-rich condition(all engines are set to run a little rich as a safety margin). I know this because I was one of the first people in the UK to attempt to commercially manufacture these devices. I sold 50 of them at £3500 each to a big biofuels company and 49 owners came back claiming fuel savings but none of them provided any data to back it up so we went to milbrook, paid a ransom and had them tested on a double decker bus, the unit was supposed to be a fuel saver and drew 16A from the cars alternator to make the gas so more load was added, it made fuel consumption about 15% worse but showed a slight reduction in emissions while it was running. So the lessons learned were that the placebo effect is vastly underrated to the point that 49 people (hauliers mostly) experienced a non existent fuel saving when it in fact made it worse which I can only attribute to them changing their driving style. I ceased with my endeavour after seeing the results but a friend of mine kept going, he made a few quid but now the proverbial cat is out of the bag and everyone's at it dot com.