My Audi A3 BKD engine code. has no boost at all. it isn't running in limp mode. i've had it hooked upto vag com and it is running perfect i have changed the EGR valve, the boost sensors, MAF, the injector seals, the fuel filter and still it has zero boost. any help would be a bonus

SOURCE: 2000 a4tdi turbo boost problem seems to goto into

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.

SOURCE: car goes in limp mode when seeing boost

Some K&N air filters are an "oil-soaked" filter and it is that oil that can cause the problem you are having. If you want to be sure, remove the filter, clear the code(s) then drive the car. If the code(s) do not return, install a non-oil-soaked filter.

SOURCE: My Audi A3 2.0 tdi limp mode/turbo overboost

i also have done a diagnostic test and it states a turbo overboost condition and it drives fine locally but loses power on the motorway and struggles to reach 80mph? any suggestions please email me at [email protected]