It's my understanding that the adblue down that pipe is designed to be at 6bar, and the adblue tank manages all the stuff to deliver that 'readyness' - i.e. not frozen and under pressure. Sometimes the pipe will have a heating jacket, and this is also controlled via the tank, and has a %age heating modulation.
When the engine stops the adblue system should suck it back to the tank, to stop it loitering in the pipes.
The adblue injector, injects under directions of the ECU, and it varies depending upon how much NOX the ECU thinks is being created. I've watched this on a graph on the Launch X431 and once the system is ready, it does it I suspect in short bursts, more bursts for more NOX suppression.
If the adblue injector sticks open, the DPF + SCR can be wrecked. i don't know why the adblue ECU doesn't detect this condition and stop, but it appears that some merrily pump on. So even though expensive, I'd recommend only getting a Peugeot one and checking that it's clean.
The Wynn's seems to keep them clean:
I would highly recommend add a small amount of Wynn's ad-protect or crystal clear - designed to stop the adblue crystalising.
The injector can be tested, it should have a resistance of about 12 Ohms and using a testlight on a battery, should just about click open just from the current on a testlight, from a 12V battery.
The pipe is undone by carefully pressing on the 'collar release tabs', and the electrical plug by lifting the tiny black plastic locking tongue.
Try to get 6mm plastic female and male plugs for this job as adblue should never be allowed to dry out. I.e. disconnect - then - plug.
Final rinsing out the pipe should be with distilled water (deionised Tesco etc), as you want to get nothing building up in there.
When undoing it from the exhaust, have a spare M4 (or M5 - can't remember) longer bolt as the clamp is 'a bit fiddly' as it includes a spring clip and is in two parts - so it's easiest to take the short bolt out and put the longer one in, temporarily, as it makes extracting it without all the bits falling out - much easier.
