A3 engine management light

I need more information to help. Use the comment button below. What colour is the light? Red is a warning light that needs sorting as soon as possible. Yellow is an information light which isn't serious.
What make and model is this for? Is this the light for the 4WD system or is it the engine shaped Engine Management light?
The Engine Management light needs looking at SOON! You will probably need to get the vehicle plugged into a diagnostic system or an OBD2 scanner. Ask at three or four garages as the prices charged can be very different. Scanning the vehicle will give you Trouble Codes that will give better information as to what the problem could be.

This could be a few things. Does it affect how the vehicle starts and runs? The obvious answer to this is to get it plugged into an OBD2 scanner or a diagnostic system to get the trouble codes. These are clues to what the exact problem is. This, Unfortunately, will cost you money. Shop around as prices can be very different between garages.
If your mostly getting Brake related lights it could possibly be an issue with the electronics in the brake management system, (ABS, Traction Control), So driving it should be limited to travelling to the garage workshop until you find out what the issue is. If You get the codes comment below with them and I'll try to help with them.

Your in south wales .With no information about the vechicle year make model .
I'm in the USA .
And have little idea of what systems are required in Europe
Sorry

Sounds like faulty coil. Check this on a diagnostic machine at a garage and you will get the answer.

Change your Engine Position Sensor, aka Cam position sensor.

Take it to auto zone and have the code's reader hooked up on it and that can tell you the problem

Throttle Actuator Control Module Performance

the eml light can only be reset at a garage with either an audi supplied diagnostic tester(5051,5052b)or a garage that has VAG com.(a diagnostic software)the latter is usually the cheapest,some audi independent garages have VAG com.

Sounds like carbon build-up in the cylinders. Take it and have it put on a machine called a "Motor-Vac"

You mentioned a non genuine cat being fitted. That in itself is not too much of a problem, however, non genuine oxygen sensors are. There is a huge difference in quality between the genuine article and cheep alternatives on the after market. I very much suspect your fault code has been tripped by such a replacement part. Exceptionally complex engine management control systems just cant recognise the electronic info thats being sent by non-gen sensors, so its a false economy to fit these. Also one thing to bear in mind for the future is to avoid using super market fuels. Its a common belief that all fuels are the same and it all comes from the same tankers, etc etc. This is absolute rubbish. Its like comparing a 20 year old single malt Scottish Whiskey with a Happy Shopper bargain blend. It may look the same, but there's simply no comparison. Prolonged use of these cheep fuels will produce long term running problems. Also dont forget, there are pre and post catalytic converter oxygen sensors on your car, so the bills can get expensive. Good luck