is there any hissing noises under boost. mine did something similar and i had a split in one of the intercooler pipes

The 'replace the turbo' line from the dealer is typical of main dealers these days.

They aren't really interested in fixing the problem, nor do they probably have any idea how. All the want to do is sell you expensive parts at a large margin and expensive labour.

Given that it works fine half the time, you only need the slightest grain of common sense to figure out that you have an intermittent fault and it can't be the turbo. A turbocharger is a mechanical machine - it is either in a functional condition or it is not. The vanes can't be damaged beyond use one day and not the next, or anything else like that.

Cars go into limp-home mode because the ECU is told to do so by some sort of sensor. So either you have a problem with a faulty sensor, or else a set of conditions are occuring which triggers the sensor. If the dealer are saying 'turbo', then it is probably because they have read the fault codes and the one which appears to be causing this problem is a triggering of the sensor which is designed to detect turbocharger failure. Then they can fit a nice new expensive turbo for you, and when you come back next week with it still happening they can scratch their heads and start thinking of other expensive parts to supply.

It is obviously not detecting wholesale failure of the turbocharger (because sometimes the turbocharger is obviously functional), but rather it is detecting a set of conditions that look like turbocharger failure. I don't claim to be an expert on how such sensors work, but it is plausible (as someone has already mentioned, through a split hose) that the sensor might detect low boost pressure and assume turbocharger failure, hence it goes into limp home mode and leaves a fault code saying that the turbo is duff.

So check your turbo hoses. Check anything else that might cause a sensor to think that the turbo has failed. Do not replace your turbo - the fact that it works some of the time is evidence enough that there's nothing wrong with it. Stay away from the main dealer, go and see an independant diesel specialist.

Also, if it has happened since new why didn't you take it back under warranty? Perhaps you did and they were never able to fix it, in which case after a reasonable number of attempts you should have returned the car as not of merchantable quality.

I can't see it being the turbo if the engine sometimes works fine. Turbo failure tend to result in a massive drop in power, more smoke.... Find a better dealer or a good independent.

Steven Quas , Hamburg