This is really hard to know what to do. Some of these check engine light issues can be a process that takes a little while to address, especially if the fault is intermittent.
Ask the dealer if they have checked TSB's, Tech-Net notes and any other technical data from Volvo. Then ask if they have called the Volvo technical hot line for assistance.
All of these things are available to a Volvo Dealership so they can address these hard to fix problems.
If you can get the diagnostic trouble codes, post them here and we may be able to provide further help.
Good Luck!
Those codes usually relate to the variable camshaft timing mechanism. The solenoid may be stuck and require replacement. One can check whether the solenoid's plunger can move when you apply voltage to it. Just pull the solenoid off the top of the engine. That should be cheap to do (0.5h labor and a new gasket). There's no reason to replace the solenoid without checking it out first. On my car that solenoid was completely stuck at 180,000 miles -- as if the plunger was welded to the housing.
The variable timing hub may require inspection and possibly replacement if it's leaking oil -- it'd be very obvious when you look at it and you see oil leaking from the top of the engine. You'd have oil in the spark plug wells in the engine head.
You can also have a gummed up engine and have insufficient oil pressure at the top of the engine. That would require taking the pan off the engine and mechanically cleaning oil passages (should be a $600-$700 job).
And you'd probably need to switch to 5W30 synthetic oil, and change it every 3000 miles. Be religious about it. It makes a big difference if your variable valve timing is marginal.
Here's the route I'd follow:
1. check if the solenoid is stuck, replace as necessary
2. while the solenoid is off, measure oil pressure at the solenoid's inlet port (and not somewhere at the bottom of the engine!)
3. if #2 comes out too low, change oil to 5W30 (filter, too!), remeasure
4. if still too low, pull the pan, clean oil passages, replace all oil line O-rings accessible from there, reassemble and recheck
5. if still too low, service oil pump
6. if still too low, I don't know -- but you must be getting adequate oil pressure at the inlet to the solenoid to operate the variable valve timing actuator, so before that happens you're not getting rid of those codes
7. having adequate oil pressure coming into solenoid, and solenoid confirmed working, if the codes come back you need to replace or overhaul the actuator (my dealer calls it VVT hub).
