Don't know much about these cars but I would have said that surely you'd find it on the 'common rail'? Don't these devices just fool the ECU into thinking that the rail pressure is different to what it actually is? So they must plug into the rail pressure sensor?
Have located the CR sensor towards the back of the engine, it's awful to get to without a lot of hassle and I'm not one for that. Also there have been issues with some devices if fitted via the CR socket.
Having spoken with people who have used and are using some of the very latest devices they have found they actually do save fuel as long as the car is driven in a like for like manner. Also the latest software inside some of the devices take account of a DPF and as such the software inside the device slightly tunes down the potential output to ensure no additional particulates are produced that cause issues for a DPF.
The key is to have the device carefully tuned for a particular car engine characteristics and as long as that is done the issue of additional smoke is not an issue for a well maintained TDCi with a DPF.
The latest software inside some of the more upmarket devices are designed to allow the car to pass the MOT emissions test with the device insitu and are also TUV approved. If a device doesn't conform to these attributes then be very wary.
Finally the issue of insurance cover. The insurance company must be notified if a car has one of these devices fitted, as they should be for any modification, otherwise the owner of the vehicle is not covered by their insurance, and were something to happen they would personally be liable for any costs incurred and could be prosecuted for driving without valid insurance. It's simply worth risking for the small incremental cost of insurance, I was quoted around a 10% premium were such a device fitted to the car.
I would be wary of these devices. Car makers have teams of engineers working for a couple of years to get the fuelling right, and then we are supposed to believe a couple of blokes in a shed can make one of these boxes and improve on the original?
Not so qxman, R&D costs the good companies £100,000s to develop the devices. The upmarket devices are not produced by a couple of blokes in a shed. As I said if a device is not approved by TUV and the like and it doesn't confirm to MOT emission standards then be very wary (leave well alone in other words).