Most generic OBD readers should talk to the engine ECU. If there's issues with the k-line, maybe other wires on the ECU have issues such as sensor wires that might impact the fuel consumption ?
If the Autel connected did you check sensor data ?
Exactly, although I called it a "special wiring adapter" to not give away the secret on an open forum. It does require "suitable" diagnostics (I presume yours has the required functions)
How to do the bonnet latch part is is well described in various threads. To actually unlock the doors you needs to access the diagnostic lines & have decent diagnostics.
@mowoh as I said in reply to your PM, I would need to see picture of you...
BTW, for the coolant pressure test, I have two of those no-name kits, and found the documentation on what car each is for to be wrong. As said above try several caps until you find one that seals properly.
I suspect previous owner(s) might have fractured number-2 glowplug in my son's TD4. The threaded part came out leaving the centre firmly in the head. The other 3 changed for new plugs, so it starts mostly ok !! Even leaving it "unclamped"...
That pic looks like the valley gasket metal clamp. The bolt is nowhere near the coolant, so unlikely to be that.
The leak is most like one of the two coolant ports on each head. If I remember correctly the inlet manifold block the two rear...
Update. Firstly stipped and cleaned the injectors, inlet manifold, inlet ports. Then cleaned out all the carbon build-up from injector-2 port in the head (other three were fairly clean but did them as well).
Cleaned out the vortex crank...