The first port of call would be a full diagnostic code read to determine what code(s) if any are present. From there a diagnostic path can be established.
You have stated that the diagnostic you had carried out said Sensor 1. That means absolutely nothing to me and the part should never have been replaced purely on a fault code! You have also stated you have had various things cleaned and checked but the fault still hasn't been cured! It sounds like someone is playing Parts Darts at your expense with no real idea of what they are doing!
I would suggest if you can't carry out the work yourself then you try a VW Dealer who has the necessary equipment. technical information and VW trained technicians who can diagnose accuartely and fit the part that is needed and not random parts unitil a cure is found!
When a Diagnostic Code Read is carried out and a Fault Code is present this Fault Code only shows the Circuit that is at fault. It might be an Air Tempereature Sensor for example. That code doesn't mean that the Sensor is faulty. It means there is a fault on that circuit. Further diagnosis should then be carried out. The Temperature Sensor should be checked for resistance against the various Temperatures listed in the Service Manual. If that tests ok then the Wiring Harness and Connectors should be checked between the Sensor and the Engine Control Module. If that all checks out ok then it leaves you with the Control Module itself that has the fault! So to summarise:
- Sensor Checks
- Wiring Harness / Connector Checks
- Engine Management Module
They are the three items that require checks before replacing a part!
You have stated that Spark Plugs have been checked. Quite knows how one would check a Spark Plug I do not know! You can check how clean the electrodes are and make sure they are not burnt or worn. However inside the Spark Plug is a Resistor. They don't exactly stamp the Resistor Size on the Spark Plug and it would be impossible to really say if the Spark Plug was ok! If the Spark Plugs haven't been changed at Manufacturers Recommended Intervals then I would suggest that these are changed!
You have also stated that the Coils have been checked but again unless access is available to the Service Manual they wouldn't have a clue as to what the resistance through the Coils should be! If the resistance through the coil packs is compared to the Service Manual then these could be eliminated. If it is found that a coil pack is ffaulty then all of them should be changed or you will end up with Cylinder Balance Issues having one new and the others old!
The other thing that shoudl be checked which you have made no mention off is the Fuel Injectors. It could be that you have a faulty Injector. It maybe an Electrical Failure or it could be an Internal Failure where the Fuel is leaking past the Nozzle and potentially giving you an overfuelling issue. Again these shoul dbe replaced as a complete set due to Cylinder Balance Issues.
The Electrical side of the Coil Packs and Fuel Injectors can be checked using the VAS 6150E Oscilloscope Function which will check the correct operation of the System.
The Technical Data for checking all of the Parts is available on VW Erwin which is available here: VW Erwin