I have a 2001 V70 T5 with 163,000 miles. It was showing faults p0442 and p0456 which are evaporative emission control system small and very small leaks. I’m new at the garage/mechanics that I’m using (my independent volvo specialist recently moved away 
) and they said they could not find any leaks, but also couldn’t isolate the vent valve. (don’t know what that means) but they replaced the hose at the purge valve as a “possible fix” for $94.00. Two days after having my car back the check engine light is on again. What is the best way to find the leak and/or repair the problem. I’ve also read in some forums that it could just be that a new gas cap will fix the issue?