p0420 code by the sounds of it
he has the integrated heater resistors and simulator on a backing plate wired up to the connector of a rear lambda so it is plug and play
the voltages do range from .3 to .6 and the average is .45, so it could well be a front lambda issue but i doubt it, i just think the ECU does diagnostics checks every 50 or so miles and checks on the averages of both lambdas at any one time to see if the cat is doing it's job
with me m8 i have a sports cat and 2.5" downpipe and i just replaced both lambdas, also since i put my catch tank back on the p0420 code has re appeared , so fricking strange
suggest trying a different front lambda and see how u get on, the p0420 code was hardly popping up then