[Batmanntexas]

Not having catalytic converters might cause a RICH DTC, but you would need someway to be able to monitor your STFT (short term fuel trims) to see if the computer was constantly adding fuel for the increased air flow.

If you were not aware just how it all works, the front two O2 sensors (Bank 1 Sensor 1, Bank 2 Sensor 1) are monitoring for STOICH AFR, i.e. they control the fueling of the motor, and are only in play during CLOSED LOOP operation, meaning they are cutout of the system at WOT or OPEN LOOP, depending on how your tune is configured. The second set of O2 sensors (Bank 1 Sensor 2, Bank 2 Sensor 2) are predominantly there to monitor catalytic converter efficiency/operation, however they also play a role in determining rich/lean condition as well. There is nothing that monitors whether you have resonators or mufflers or don't have them.

If a mechanic told me that not having resonators was causing my RICH DTC, I'd find another mechanic in a hurry. A resonator is a hollow tube of a certain length and diameter, engineered specifically to cancel out those resonant frequencies that would otherwise cause excessive drone within the car. They have a negligible effect on exhaust gas flow.

From your initial post, what the Dealer is calling the resonator, sounds like the actual catalytic converter. A lot of Forced Induction guys, myself included, have hollowed out the catalytic converter so as to not cause problems down the road, yet give the visual appearance that they are still in place.

If that is indeed the case, you will definitely need a tune to get rid of the CEL. If done properly, it will still pass an ODBII Emissions test, unless they take a tailpipe sample.

-Scott