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Check the wires in the rubber boot by the door hinge. The wires might be fractured & making intermittent contact.

I don't have a picture but it should be fairly obvious. There will be 4 coils in a row on top of the engine with multi pin wiring connector to each. Perhaps someone else reading this will have a picture.

The fault code shown by your reader specifically says " Ignition Coil Primary Circuit Low " which could be either a wiring fault to that coil or the coil itself. Check the plug-in connection to the coil as I mentioned above. The coil itself could also be intermittently going short circuit internally due to heat. If you swapped the coil for No3, then checked the code reader when it next failed you would know if it was a wiring fault or a faulty coil.

His car has gone through 2 MOT's fine, mine has passed 2 MOT's also with one fitted. The only way they would see a problem during an MOT is that there is no longer an electrical connector in the EGR. But it is quite well hidden from above & below as long as the undertray is fitted. Any work on the EGR is difficult due to very limited access, which is why it costs so much in labour to replace. To my mind it's going to fail at some time in the future, so being pre-emptive isn't a bad thing, especially if failure leads to you being stranded somewhere with a non-starting engine.

The EGR is disabled by the simulator, so the cooler then becomes redundant, although it will still have coolant flowing around it. It sounds like your EGR is clogged with black exhaust particles, eventually it will fail & cause limp mode. My son's one on his Tiguan failed stuck open & prevented the engine from starting. He's had no further problems since fitting the simulator over the last 2 years.

" it started to shudder (coil I thought to me self). " From the code recorded by your code reader it looks like your hunch was correct. The fault code is indicating either the coil is faulty or the electrical connector or wiring has a poor connection. I'm assuming coil D is coil No4 which I believe would be the one furthest from the flywheel. I would inspect the electrical connector to coil 4 & check for loose wires or burning on the connector. Perhaps check all 4. If nothing suspicious found, I would change the coil pack, they aren't usually very expensive.

Yes it tricks the ECU into thinking the EGR is still functioning, when in fact it has been un-plugged from the cars wiring. The ECU doesn't need any re-mapping. The DPF still function exactly as before but with much less particulates going into it, so if anything has a much easier life. NOX gas from the exhaust will increase, but that isn't tested at the MOT, so the car will pass no problem. The unit ( EGR Simulator ) is just an electronic box with 3 cables. One cable plugs into the plug removed from the EGR. The other two cables plug into the MAF sensor on the air filter pipe & the plug removed from the MAF sensor.. I've only fitted to the 2l engine, takes no more than half an hour. Remove the air pipe to the turbo for access down the back of the engine. You need a mirror to be able to see the EGR plug that has to be removed, that is the hardest thing & once located & removed, the rest is plain sailing. Use plastic ties to keep the wiring neat & away from anything hot or liable to rub. I'm pretty sure the 1.6 engine will be almost exactly the same. Look at the sellers add which shows the work required to fit. Once fitted you can clean out the throttle valve & inlet manifold of all the black EGR gunk & know it will stay nice & clean, as the engine will no longer be ingesting it's own excrement. The oil will stay cleaner for longer as well.

Look at this post below for the easy & cheap fix for this problem.

It just tricks the ECU into thinking the EGR is functioning normally. If anything, it helps the DPF as less soot is produced by combustion. The combustion temperature is raised as no EGR gas to lower it. The ECU is programmed to disable the EGR when a DPF regen is required, so no problem there. The MOT doesn't check NOX gas, just soot level, so no problem there either.

Fit one of these & forget about the EGR. https://www.ebay.co.uk/itm/183869556154?hash=item2acf7ac5ba:g:OaAAAOSw-DRdHK14 The EGR isn't fit for purpose IMHO.

Could also be fuel returning to tank overnight replaced by air. Try switching the ignition on several times without cranking which will run the in-tank pump. Then crank the engine on the 3rd turn of the key.

Have a look at the map of VCDS owners, there may be one close to you will probably be happy to scan your car for a few beer tokens. https://www.google.com/maps/d/viewer?msa=0&mid=1Td73_uUUqscV3nRm5br_o89PmBU&ll=52.396527458722325%2C-1.8395237437499468&z=7

Quite likely to be the EGR stuck open. If it fails again, block off the EGR pipe & try starting again.

You could just unplug the electrical connector from the Compressor to stop it operating the Compressor clutch. Then get someone to run a diagnostic to see what is putting the engine into limp mode. The 1.9 Tdi of that era is pretty bullet proof. May get a few more years out of it.

Another here although seems a bit expensive. https://www.ebay.co.uk/itm/184219442814?hash=item2ae4559e7e:g:l~gAAOSw07JedTB3 The company selling this gearbox is a specialist at repairing these boxes. So might be worth speaking to them to see if it might be worth repairing yours.

This breaker has PBE gearbox. https://www.ebay.co.uk/itm/255125503588?fits=Car+Make%3AVW&hash=item3b66aa3e64:g:eK4AAOSwOQthN2cr

Diesel fuel that is left in the tank for a long while ( It had been sitting for 8 Months waiting to be sold ) can develop an algae growth. This then blocks the fuel filter. If the problem returns re-check the new filter.

That sounds like your pressure sensor is goosed. There should be a membrane between the two sides of the sensor that stops exhaust gas exiting on the atmospheric side. Seems like that is broken allowing the exhaust gas to escape & will have cooked the sensor.

Why not block off the EGR then see if it will start. It might put the engine into Limp Mode but will give you a result one way or the other. I know from my son's Tiguan that a stuck EGR can cause a no start situation, but that was on a 2l with DPF.

Could just be an intermittent stick coil breaking down causing it to run on 3 cylinders. Get it scanned when next running rough.

You can't just swap the ECU, it has an immobilizer that has to match with the instrument cluster. Does the engine run? If so, get a diagnostic run on the car with VCDS. That will give you/ us an idea of the problem causing the engine light & probably has nothing to do with the ECU, which is normally a very reliable part.

I've got a diagnostic cable I don't use any longer which I can post to you FOC. Used to use it on my old B5 Passat, but now have a newer cable needed for the Superb. Happy to post it to you if you PM me your name & address.

Check all the rubber boost pipes between the turbo/ intercooler/ inlet manifold for leaks at the joints & splits in the rubber. Any leak will loose boost pressure hence underboost. Splits are sometimes difficult to find, but usually leave black marks from sprayed oil mist onto adjacent items.

I would agree that it looks like your EGR is sticking & causing loss of power. If it gets worse the DPF will start to get clogged as well. Suggest if you have the CAY engine you fit one of these :- https://www.ebay.co.uk/itm/181994477873?hash=item2a5fb75931:g:xEUAAOSwhvFZAvHt This will disable the EGR & restore the engine to full power again. No problem with MOT either.

This one is on eBay at the moment. https://www.ebay.co.uk/itm/323706905373?hash=item4b5e6f631d:g:8PUAAOSwfINccS8C