help with a4
 

Hi im new on here i have a a4 sline estate 1.8t 2006 (74000 miles) about 3 weeks ago i had the emission control warning light come on, run ok but misfire on ticover fault codes 16891 and 17544 i took to garage who said it was air leak replaced 2 pipes got back then 4 days later low oil pressure light came on, took to garage who remove slump and replaced pick up screen, got back again now getting emission light again fault codes 17700, 17544 and 16891. Took back to garage spent 2 hours going over said could not find any thing, got 60 miles out of it then light back on same faults runs ok just rough ticover and smells hot (temp ok) but garage said would smell for while. I have read could be non return valve could this be right or not or have does anyone have any ideas what could be happen cheers