First, Don't use the after-market one size fits all O2 sensor. Second, Make sure the Cat is working. To test is simple, use a heat gun and make sure the outlet right after the Cat is hotter than the inlet, right before the cat. Third, have the shop make sure to test the O2, most sensors are not the problem, the shop just does not know how to test it. Fourth, the shop has to measure the current to the EHA and set the mixture control at the same time. Do that, and if everything is still not working right, drive it if you have not yet. FYI, the Cat will be ruined if it is running too lean, or too rich.
GVB