Ever since my last service, I have been having a problem with outside smells (mostly other cars' exhausts) getting into my car thru the A/C vents. Sometimes, its smells like an exhaust is ducted right into my car. I don't think my my own car's smells bc it mostly smells fine.

Does anyone know what could be causing this problem? The filters were (allegedly) changed at the last service. Is it a problem with the Automatic Re-circ function? Is the A/C system supposed to turn on Re-circ automatically if it detects the outside air is worse than normal? Thanks.