Hi. I have a 2008 c6 avant 2.0 tdi. Every now and then I get no boost to the turbo. I have removed the turbo and rebuilt. I have replaced a soft and squidgy pipe from turbo to intercooler. I have replaced the boost control valve and all the pipes looked in good condition. It crops up every now and then. If I drive like a granny it’s very rare but if I boot it it’s more prevalent. It doesn’t produce any fault codes. Any ideas please? Could it be the actuator?  Not sure what else to try. 

You must be knowledgeable and spanner handy if you have rebuilt turbo. Given the time to remove and the likelihood that it could potentially fail again, I opted for a new unit from https://turbo-diesel.co.uk on my old A4 Allroad.

Back to your issue. The actuator can be be tested with a hand held vacuum pump if it holds pressure. Trouble is, it’s pretty specialised to remove actuator and reconfigure. On new units they are set up from factory, so basically plug and play.

Did you renew the oil feed line to the turbo? The union nut at the turbo is known to be problematic and can weep if cracked off. In addition, did you renew the gasket to the exhaust?

What are you using to read codes? VCDS would be software of choice and I’m sure would log actuator fault if low boost.

Yeah it’s all just nuts and bolts at the end of the day!! 
The actuator only seems to have a threaded nut to set it so I think if I could find the tolerance or measurement online it’s not too tricky. There were no issues with the oil feed and no leaks. Gaskets were all fine except for the one between the manifold and the EGR so I replaced that one. I’m using an Ancel VD700 to read codes etc but it’s not throwing out any codes. As a side note I’ve driven 300 miles today and it’s not happened so it’s one of those intermittent things. I’ll find it at some point I guess!!