Pontiac Montana sometimes doesn't start after getting gas

Hello, P0449 indicates evaporative emissions system vent solenoid malfunction. The P0455 means an evaporative emissions system leak. P0171 indicates a lean condition, as you stated. P0101 relates to the mass air flow sensor. Diagnostic trouble codes only serve as clues to help guide a technician towards finding out what's wrong with your vehicle -- or at least what is causing those codes to set symptomatically. The problem is - there are dozens of possible root and contributing causes to both those codes - and the performance problem you are sharing. Based on your concern description, I'd be looking for presence of an air intake concern, a possible vacuum leak, something that leads to an air-fuel mixture concern. This is something you cannot really guess at; hands on testing and monitoring of data from various sensors and components is needed to figure out what's going on. The evaporative emissions system issue could possibly be contributing to the hard starting issue as well, if there is a fuel tank vacuum issue present. All of these things are total guesses until testing is performed. I will say that since codes are getting set, there is likely useful data to assist a technician in figuring out what's wrong with your vehicle. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops