C30 Engine : 2008 C30 T5 with error code 640C

[2.4TSE]

My wife's C30 T5 went in for rectification of a trouble code that I can't remember but which turned out to be caused by a torn diaphragm in the PCV chamber. This was fixed by replacing the diaphragm, physically getting to which entailed removing pipes that includes part of the induction system. Also changed before realising that it was a torn diaphragm was the MAF sensor.

After repair, the code was gone - but replaced by code 640D and an engine down on power. This code indicates 'Turbocharger (TC) Control System, Signal Too LOW' and arises when the ratio between the boost pressure measured before the throttle and that which is calculated after the throttle is incorrect.

The mechanic replaced the boost pressure sensor at the end of the intercooler and this code went away.

It was replaced by 640C, 'Turbocharger (TC) Control System, Signal Too HIGH' - so the opposite of before and indicative of a boost leak. I asked that the TCV be replaced anyway as it was the original at over 150,000 miles, but error code 640C remains.

Is the cause likely disturbed pipework as a result of the other work, or something else, please? I am trying to arrange a smoke test to look for a leak that way but so far both me and the mechanic have been unsuccessful at finding someone locally enough (I don't want to drive the car for miles and miles in its current state).

Cheers!

[2.4TSE]

The cause is the turbo failing to hold consistent boost. After the sensors replaced and a clean bill of health on the integrity of all pipework and hoses, the suspect is a failing recirculating valve (the one that allows excess boost to bleed back in to the induction system).

The question is, can this assembly be purchased without replacing the turbo itself, it looking to be perfectly fine?

I will start to look, but experienced comment here would be helpful.

Cheers.

[2.4TSE]

Closing the loop for anyone finding this thread in the future.

I was unable to source the pressure relief valve in isolation so ended up having a reconditioned turbo unit fitted. This is the entire assembly of exhaust manifold, turbo, and pressure relief valve purchased as one entity.

Problem solved. We now have a properly performant C30 T5 without any error codes. With new MAF, boost pressure sensor, turbo control valve, and hours and hours of diagnostics on the way to buying and fitting the turbo assembly, I also have a somewhat lighter wallet.