It could be many things. I just recently, as in 2 days ago fixed my P0455 large evap leak/no flow code. I will give you the order of the things that during my 6wks of researched, helped me fix this code. Following are the things, in order respectively, that go wrong to throw the P0455 code.
1--Evap system hoses
5--Canister valve(some people call canister purge valve)
A crack in an evap hose is the most common thing. I would definitely check the two evap canister hoses. These are easy to fine, one being the J hose, which is in a J shape, and connected to your canister and the other end to a valve. Then the Canister Evap hose, which is 18" or so long with a 4" L on the end of it. It connects to a hard plastic or metal hose that can easily be reached by removing the rear passenger side tire on a 98 S70 turbo. Check these 2 first. I found that the cracks in both of these hoses were where the clicks were or close to them(many old style clips causing this). Check these too, again they will both be connected to the canister valve that is right beside the charcoal canister on the passenger rear side.
change these if cracked and reset the code by pulling the battery terminal for 30 min, connect again and the whole system will be reset as well as any other hidden codes that you can't see that the system was hiding.
Both of these were cracked with me, so if the code comes back the next thing you want to do is look at those 3 hoses under the car there by the canister that run up to the front of the car, check them with a good flash light, all sides, and take your time. A P0455 is a GROSS leak, which is good because a leak in a hose will be big enough to see.
Reset again, if the codes come back like they did with me, open the hood, look under the radiator shroud and you will see a valve that has 3 connections on it, 1 electrical, and 2 hoses, one pointing down and the other horizontal. Check the hoses there, as well as all elbows that are on evap hoses for leaks, now pull that valve off and test it by going to O'Reileys for $30 or Autozone and getting a vacuum tester that measures in hg and make sure that both hose ports hold vacuum for at least 20 seconds. If it doesn't it's this purge valve that is faulty, people list all kinds of ways to test it, but I bought a new one cause my was faulty and it makes no sound, no pulse, no click, no signs to tell it's working except the vacuum test. After you replace the purge valve, reset codes the same way and drive as you normally do.
If the code comes back, first thing in the morning go out open your hood, has to be on a cold start up, start the car, then go and put your hand on the biggest hose(air pump hose) that you can see on the right hand side of the car and going under the battery tray, if your air pump is bad, then you will not feel any pulses of air through this hose, if it works you will definitely know hands down. NOW, that pump is only on until your car has sufficiently warmed up enough to clean the fumes created by the car then it will shut off. I say do this next because it's logical in the order of what is most times wrong, if not hoses, or if hoses but the code comes back, spend the money on the vacuum tester and test the purge valve, if code comes back next is actually the check valve at the back of the engine and fire wall, mushrooms shaped with a hose that goes to the air pump connected to it and a wire going to the solenoid I think. Anyhow, if your check valve is bad your air pump will blow soon after....so if air pump good, check valve is good, if you want to double check, take your handle vacuum tester from oreily's and guess what? yep, put it on the big hose opening and see if it holds vacuum, if it don't it's the check valve recently going out, and you'll save the air pump, but if you can feel the air pump pulse that hose on a cold start, your check valve and air pump are usually fine.
So as with my code, I replaced the J hose and evap hose going to the charcoal canister valve at the rear, codes came back, put new cap on, code came back, faster!, pay attention to this, checked all evap hoses, verified they were good, then tested purge valve under radiator shroud and replaced, verified air pump and check valve were fine with cold start test, now it's most likely from here your charcoal canister valve that some people call your canister purge valve, it is what the J hose and 18" Evap hose you just replaced connect to. Replace it. Reset code same way and watch, if it comes back, hands down it's your solenoid, that is considering your evap hoses are fine, the purge valve, and check valve are fine and your air pump is good.
With my car, I did J and Evap to canister, codes, then new cap, codes but faster, so old cap and new purge valve in front, codes--but when checked P0108 manifold, or MAP, reset and changed canister purge valve, codes, but this time P0133 02 sensor, that told me that no matter what my evap leak was gone and I just had the MAP to deal with which can throw a 0133. So I put the other canister purge valve back on since it originally threw the P0108 and then replaced my MAP SENSOR and haven't had codes in 150 miles when I would get the code every 13 miles before.
This sounds TOO expensive, but it's not, go to a junk yard man, half them guys don't know what you have is or what it's worth. Just know what your looking for and where it's at by doing a google search and go pull parts man, get you an air pump check valve, solenoid--they are connected by wire, purge valve, air pump, charcoal canister and valve, then follow the above process, all of that won't cost you over $100 and you can trouble shoot that way.
If you don't want to do this, go to volvo or a shop that has VOLVO'S SOFTWARE, and have a full diagnostic as well as smoke test done. They can test things separately and have more precise info, the smoke test will tell exactly where your emission hose leaks are at. If you have the time, or need to save the money, it's worth it. I new nothing about this used car I bought 3 months ago and now I'm pretty good with the whole system.