You’re probably already aware of this information, but I'll leave this here just In case.
- Leaking or contaminated fuel injectors
- Low fuel pressure or running out of fuel
- Leaking evaporative emission (EVAP) canister purge valve
- Exhaust or intake air system leaks
- Exhaust gas recirculation (EGR) system
- Positive crankcase ventilation (PCV) system is leaking or the valve is stuck open
- Ignition system
- Incorrectly seated engine oil dipstick, tube or oil fill cap
Others have reported that running a can of Seafoam through the system cleared up the code. I don’t see this being a solution for you unless the code is coincidental to your exhaust mod which is dubious for me, but it wouldn’t hurt to try.
Work your way through this list, and see if you can run it down. If not, you may need to take her to a shop for the solution.