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Sounds like it's coming from the EGR then. You can fit one of these :- https://www.ebay.co.uk/itm/183869556154?hash=item2acf7ac5ba:g:OaAAAOSw-DRdHK14 as long as you are ok with the extra NOX emissions.

I have a 2l Diesel 140 Estate which I have corrected the on board computer with VCDS. Previously it was about 8% optimistic, now it is fairly accurate, slightly under reads if anything. Did a journey from London up to Cheshire 2 Sundays ago, M1 / M6 170 miles. Set the cruise to 62MPH on the Motorway which was 90% of the journey & arrived home with 69 MPG for the journey, which I thought was quite good.

There are 2 sources for the clicking noise of stripped plastic gears. One is the EGR at the back of the engine, the other is the throttle valve on the inlet pipe just behind the radiator. Check which is making the clicking noise before you go further. Either will result in reduced engine power.

I have had a sort of vibration during acceleration, don't know if that is the same sort of thing as lumpiness. It wasn't always there but could be felt throughout the car intermittently. I had the bottom dogbone engine mount changed. The rubber bush had deteriated ( car has covered just over 200k miles so not surprising ) which allowed the engine to shunt back & forth too much. Only £16 for a new one & solved the problem.

This worked for me :- https://cardiagn.com/skoda-superb-ii-b6-workshop-manuals-owners-manual-wiring-diagrams/ Wiring diagram is down towards the bottom. Other workshop info also on there.

If you remove the small coolant pipe near the top of the coolant reservoir then poke it in the reservoir top. Start the engine, coolant should circulate through the pipe & into the reservoir. That will prove the pump is working. If no coolant comes out, either the pump is defective or the small pipe has a blockage.

" Trouble codes: P305200 - Starter activation, return message terminal 50 Short circuit to B+ static " Terminal 50 is the output from the ignition switch to operate the starter. I would start at the ignition switch & check you have an electrical signal leaving the switch when turned to the start position. From there I would guess it goes to a relay, then on to the started solenoid, but you will need a circuit diagram.

As you've got the turbo off, have a look at this video about the turbo mechanism. The control ring for the variable vanes is probably sticking due to a build-up of carbon in the mechanism. It's not much of a job now you have it removed to inspect & clean the mechanism.

What is the engine code of your son's car ? You will probably need an EGR emulator/ simulator or the ECU will detect the change & put the engine into " limp mode ".

It's sometimes a sign that the clutch isn't disengaging fully. Can you select reverse easily when the engine isn't running?

What is the engine code. You might be able to fit an EGR simulator/ emulator depending on the engine, then forget about the EGR & any faults it has.

If you are getting this overheat warning when the engine has just been started ( & obviously still cool ), you've had the coolant temperature sensor changed, I would suggest you have a wiring fault somewhere in the engine bay loom which is giving false data back to the engine ECU. Have a good look around the coolant temperature sensor plug & local wiring for damage, then trace the loom back towards the engine ECU looking for any chafing against anything adjacent to it. Check the engine ECU electrical multi-plug for any corrosion or moisture.

This seems to be the correct thermostat for your AQW engine. https://www.ebay.co.uk/itm/234340925912?hash=item368fcec1d8:g:iq8AAOSwuIxhvYdh As you say, it is rated at 88C.

So yes, just carry on as you did before. The regenerative braking has nothing to do with your friction brakes. I also often use gravity to bleed the brakes some times. As you say, it is slow, but there is no possibility of introducing other problems some of the other methods do.

I think this engine has a coolant cooled intercooler. These have been known to leak in the past, although it isn't a common fault as far as I know. If it is leaking internally, the coolant will be burnt in the engine & exit through the exhaust. The amount you are loosing probably wouldn't be noticed in the exhaust.

My experience with Halfords MOT's is that anything failure wise is blown up out of all proportion to maximise their profits. If it were my car, I would order a new brake fluid cap from Skoda. Get the brake fluid changed & all brakes bled by a different MOT garage. Ask them to look at the rear discs & change them if necessary, then get them to MOT it. I don't think they will find any other problems with the brakes.

I think if the tank is three quarters to full the tandem pump copes ok, but if quarter full or less it may well struggle to get full pressure.

" as soon as I opened the door charging went up to well over 13v " Check the wiring running through the rubber boot by the drivers door hinge. They can break, loose insulation & short out.

Have you checked the in-tank fuel pump. Could be cutting out, then a bit of vibration restores it again for a while.

I think it is pointing to an air leak in the air pressure pipes between the turbo/ intercooler & inlet manifold. Look for any sign of black sprayed oil residue on any adjacent surface which will be ejected from any split or leak. Also check the throttle valve on the inlet manifold. They get covered in black sticky EGR crud & start to jam up, although your fault codes don't really support this. If you remove it to clean, remove the plastic cover on the side as well. It covers the electronics & drive motor. I have found some to be completely full of black conductive engine oil in the past which of course shorts out the electronics. Give all the electronics a good clean with something like brake cleaner including the electric motor.

" More info from this morning: No change at first, but after opening the bonnet (irrelevant I am sure) the dash lit up, the driver's door started to function and the car started. " I'm pretty sure the relationship between the door module & dash both not working at the same time won't be irrelevant. They may well share a common power source. But without a wiring diagram I can't help further.

Have a look at the EGR topics recently running in the Superb2 section, probably be the same engine as yours. It sound as though your EGR is starting to fail & will only get worse. An EGR Simulator is a much cheaper & permanent solution.

If the dash is dead the car won't start. The dash also has the immobiliser chip in which has to communicate with the ECU to allow the car to start. If the dash has no power to it, the ECU won't allow the car to start. Try banging on the dash top to see if any response. Could be a loose connection.

What is the engine code ? Are you sure it has a vacuum solenoid on the EGR ?

Plenty available through eBay. https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=TH575+thermostat&_sacat=0&LH_TitleDesc=0&_odkw=vw+1.2+tsi+engine&_osacat=0&_sop=15&_oac=1