Error codes after installing magnaflow cats

[PzYkED]

Sure could use some input here. Today I got my 04 H2 with 77k km on it back from a local car guy/welder that welded on some magnaflow cats to my original exhaust system. Turned out that both original cats was completely empty (no wonder i didnt pass the emissons test).

I have the diablosport trinity and ran the diagnostic tool after the new cats was installed, i got the codes P0135 and P0155. These codes never appeared before the change of cats. Erased the error codes, checked the connectors and drove around a bit. But the error codes pops back up.

Isnt it highly unlikely that both these sensors would go bad at the same time?
I didnt think that welding on new cats would destroy the sensors, but it sure seems like a possibility now.

Any help much appreciated