Welcome to volvoforum.com. You are correct that a P0172 usually indicates fuel mixture is too rich. Basically an oxygen sensor has detected a rich condition as a result of too little oxygen in the exhaust.
What symptons are you having? Have you replaced anything yet?
A fuel trim code usually is caused by too much fuel being delivered to the engine because of a failed mass airflow sensor or vacuum leak. It's impossible to diagnose your problem on the computer but if you go to your local dealer and pay the price for actual real diagnosis, you'll probably be many dollars ahead of what you will throw at it otherwise.