[jb92563]

With no codes the car thinks everything is within acceptable parameters.

Check the cheap things first if it is the emissions out of limit and not just a faulty sensor:

Check the engine air filter.
Clean the MAF sensor with ONLY a MAF cleaner.
Check for air leaks in the induction system and all hoses connected to it.
Check for Exhaust header leaks.

Did they identify which lambda sensor was reading too high? A faulty sensor?
There are 4 sensors and easy to replace if they are known to be bad, but a little pricey if you have to do all 4.

Its a common error to replace a sensor when it is not at fault but the reading was out of spec due to an engine air/fuel issue.

Without a code or diagnostic reading of the long term and short term fuel trims its hard to figure out what is going on.