1998 volvo failed emissions for trouble codes present: P0172 P0455. What to do now

Code P0172 relates to the engine running too lean.
Code P0455 indicates that a leak has been detected in the Evaporative Emission Control System.

P0455 is most often caused by a a problem with the gas cap. It is either incorrectly installed, damaged or just worn out. Other possible causes are a leak in the EVAP system caused by damage or a loose vacuum line.
To solve this first check your gas cap for any damage or wear. For the EVAP system check under the hood. Listen for any vacuum leaks or for the odor of fuel. Check all the vacuum tubes and hoses for breaks in the line. If you find any repair or replace them.

The cause of P0172 is usually associated with a bad O2 sensor but this is not always the case. The causes could be a vacuum leak, an issue with the Mass Air Flow sensor (MAF) or some type of restriction with the fuel system.
The solutions are to inspect the inspect all vacuum and PCV hoses (which you have already done). Inspect and clean the MAF sensor. I am unsure what type engine you have but this will be located somewhere between the intake manifold, possibly on the large black air intake tube.
Finally check the fuel system for leaks. Also the fuel pressure should be checked. You should also run a good quality fuel injector cleaner through a couple fill ups.
After you have done all the simple tasks, (inspecting the gas cap and inspecting the vacuum line and hose on the EVAP system replacing if needed and the fuel injector cleaner. Get your codes reset and wait to see if they come back. If so I think your next step is to replace the O2 sensor. If this still does not solve the problem you will most likely need a complete engine analysis before you can continue.
If you get to that point you should re-post here with the results of the analysis before spend a large sum of money.
I hope this helps and good luck.

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