We sold two versions - one for petrol injection and one for diesel injection.
We knew they worked so we came up with a sales strategy of placing one in a workshop FOC except for the chemicals used. If they liked it, they bought it - if not, we picked it up.
We only had 2 rejections and sold over 100 (before I left).
One dealership was having problems with detonation and rough idle so we set one up. They put every one of these vehicles through the service on every service. They bought 2 machines after three weeks.
It was knowing what the machine could do and marketing the "service" correctly to the appropriate customers.
Sure its not a "fix all" but it worked impressively on those vehicles that required it.
Some workshops even stated that customers had asked for this cleaning service every service as the initial clean was so impressive. Most workshops talked them into one clean per year which was every second service rather than one per service. Most were paying the kit off in 6 months or less.
I will add one thing though - much has to do with the correct chemical cleaner, not the equipment and people who changed to a cheaper chemical reported less satisfactory results and came back to the original chemical.