In the course of all the filter changing, could you by chance have knocked the line off the shut off solenoid? I know it happens often on the vacuum shut-offs, the vacuum line gets bumped or knocked off during a filter change, then no stopee. Knocking a lead off the solenoid would probably give you your fault code, may even affect engine performance and would definitely cause a fail to shut down.
Mind you it's just a guess.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.