How to solve anti pollution fault on a 2005 peugeot 307 xsi hdi?

Hi all, i own a 307 xsi hdi and the engine management light and anti-pollution fault is constantly on. As a result, car has lost its power. Turbo works for about 3 minutes of starting the car and cuts out. This has gone on for about 4 months, spent loads of money but problem can't be rectified by mechanics. Can anyone please help? thanks