I hope someone can help me, a couple of months ago my MIL light came on, upon inpction i was told the the Catalatic convertors are blocked. It is way to expensive to replace so i decided to cut them out. As a result the mil light stayed on. I decided to fix this, by installing Mini Cats. This basically emulates a cat and also move the O2 Sensor out of the direct exhaust gasses. The light went of for a while only to return a day or to later, witth error codes P0138; P0158. I was told the the rear O2 Sensors, left and right after cat, were faulty and I replaced them. I reset the error codes and al went well for about two days, only to get the exact same error code again? My questions:

Has anyone installed a Mini Cat? Does it work?
I have now been told the front O2 Sensor are also shot and should be replaced, but the error codes does not show this. Is this possible to affect the rear O2 Sensor?
What other suggestions as to what can be causing this?

I am desperate. I drive an 2003 Jeep Liberty (Cherokee in South Africa) 3.7 V6 about 105 000Km on the clock.

Hope this is the correct forum for help.