I'm late to this thread and have skimmed through it, so apologies if this has already been covered.
Turbo underboost P029900 is a bit of a bugger as it could be many things.
I had this fault with no discernible loss of power at all. Not sure exactly what variant yours is but on a twin turbo it can be many things - the obvious culprits are dirty fuel filter or the tmap sensor on the intercooler. The latter is a swine to get to (bumper off, remove radiator bracket/brace but once you' re in there, 1 screw)) but it is a relatively common failure. The other is the sensor on top of the engine on the induction side that goes in what most of us would still call a rocker cover: it can fail and/or get totally encrusted with gunk.
Other things to check are the turbo control valve - start the car, let it run, turn it off and wait a few seconds and you should hear the car "burp" and see the actuator rod move. And of course, ensure there are no air leaks.
You can read the whole saga here:
https://www.volvoforums.org.uk/showthread.php?t=309996
Or, simply cut the to end for the parts numbers:
Quote:
The original part was 31303974.
This was replaced by 31355462.
The Bosch part is 0 261 230 294 which is hard to find and some say it is interchangeable with 0 261 230 293, others say not.
Eventually I found a Febi Bilstein part 103494 which says it is compatible with 31303974 and 31355462 - it even says it on the box.
I opened the box....it is a Bosch part 0261 230 293
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