m.o.t fail, lampda reading

[love206gti]

Hi, can anyone help, i've failed my m.o.t on lampda reading at 1.056, should be 1.030 or under, i've had centre pipe exhaust changed due to crack in it and a new lampda sensor fitted, already had a new kat and back box done and now on re-test it still failed but reading has improved to 1.034, but now mechanic was stumped as didn't know what else it could be?????????????