A leak would generate a CEL, but you should get it fixed regardless. Where is it? You have both the primary and secondary stock cats still on? You could also have a faulty O2 sensor among about 1000 other reasons.
08 E90 | Jet Black / Black | 6MT
Mods: RAC RG63 w/ Pirelli Corsa 255F/285R | Akrapovic Evo Exhaust | OEM Blacklines
Winter Setup: OEM 18s w/ Nokian Hakkapeliita R 245/40-18