I have 1999 Ford Windstar and before smog had P0174 lean air bank and checked online suggested spraying mass air flow sensor with cleaner and cleared code and smogged. ran for 6 months and get P0174 and P0170. Need to replace mass air flow sensor,but what is fuel trim malfunction? Is that included with mass air flow sensor or separate problem. Van would not start and cleared codes and sprayed with cleaner and ran for week but now wont start and backfires.