How to fix EPC light ON from AUDI

In your case the turbocharger is responsible for engine losing power.
The car goes in "limp home" mode!!!
After scan the EDC-15P engine control unit with a VAS, VAG or KTS -BOSCH diagnosis tool you will find this: Fault code: P1557-Turbo Boost pressure control exceeded
1. Engine stopped and ignition switch off. Check all pneumatic connections and hoses between turbocharger actuator = pressure unit for boost-pressure control, boost-pressure control solenoid valve, vacuum reservoir, EGR control solenoid valve, intake-manifold flap solenoid valve, EGR valve with throttle - part of intake manifold. Also the vacuum connection between tandem pump and brake booster. If you find something wrong replace parts. If not go to step 2.
2. Extract the hose of the turbocharger actuator = pressure unit for boost-pressure control part of turbocharger. Instead of the original hose you must place another 1 meter long hose with the same inside diameter, and then you check to inspire yourself the air from the other one extremity of this hose. The mechanical connecting rod of the turbocharger actuator must have a smooth and whole motion. If you can do that with your mouth, then you must replace the boost-pressure control solenoid valve. If you can not reach this with your mouth, then you go to step 3.
3. This is the most difficult work. The problem is that the soot particles deposits inside the turbocharger plugging the variable nozzle geometry mechanism = adjustable vanes of the turbine. If the turbocharger actuator is not able to adjust the turbine vanes the charge air pressure increase too much and ECU (engine control unit) go in "limp mode" = engine protection software. As a result the "limp home" mode engine still running until you turn the engine off (ignition switch off) and back on when the "limp mode" is deactivated, but the fault still remain in ECU memory!
4. You must be able to extract the turbo from the engine and then to disassemble the turbocharger, clean inside adjustable vanes mechanism and refit all.

Needs to be reset with a scan tool.

The best ideea is to scan your Engine Control Unit with a VAG-COM, Bosch KTS, ... diagnosis tool. If you have one of these dedicated scan tools you will find definitely the engine fault code-"P...." and the clear description of the error. Also you can try to erase this error (or these errors) from the Engine Control Unit memory. If errors are "sporadic" type you certain have permission to erase them, also for some kinds of "static" errors. Because your Engine "MIL" - Malfunction Indicator Lamp keeps coming "ON" and "OFF" you have a big good hand to clean errors from Engine Management System memory. If you don't have any diagnosis tool, you can also try something else. For check up if the engine have a proper turn, you can successively disconnect each spark plug coil from the engine wiring system, engine idle, car stationary. When you operating this you must be free of chassis ground (battery ground)!!! When you disconnect a spark plug coil and the engine speed (rpm) drops, that means this coil works properly. When one coil disconnected and engine speed (rpm) still remain unaltered, this coil (or spark plug) is out of order. First you can change only the spark plug and check up again. If no changes the coil is defective and must be replaced! My recommendation is first to change all spark plugs on the engine, and after that check up coils. Please Rate my Response! Thanks!