E320 2001 Check Engine Light on Catalytic Converter
 

The Cat made rattling noice so I replaced it(both sides) with an aftermarket Cat at a small shop. A month later, the Check Engine light came on while driving on the highway. I took my car to the same shop and the code read P022(Main Catalyst Efficiency Below Threshold, Bank 1) and P0432(Main Catalyst Efficiency Below Threshold, Bank 2). The tech cleared the codes but didn't do anything else. I continue driving the car for another week in the city only and nothing happend until I got on the highway and the CEL came on again.
I don't know what to do at this point. My car just turn 100k miles, so the warranty was exhausted. The MB Cats are about $1500+labor. Could the output and input to Cat sensors defective? If I have to buy another Cats from the MB, should I buy the Cat and the pipes or just the Cat?
Please help!! as my budget are getting very tight.:(
Thanks for all your supports