Do they test the 2500rpm first, then idle?
If so, you might need a new O2 sensor. Even though not registering as bad, an older sensor won't allow the system to normalize when dropping to low idle from high as quick as it should.
'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.