post cat lambda sensor
the first lambda controls mixture, the cat simply reduces emissions
i heard of ppl that remove the cat and do not get lambda sensor codes
as for over 5k then the engine is in limp mode and it is stopping the engine from damaging itself [major sensor fault]
you need to check the voltages of the post cat lambda to see if it is pulsing and within the required range
same applies to the pre cat lambda
if the 2nd lambda is pulsing every second but is throwing out erratic voltages similar to the first lambda then consider it a cat fault, anything over .9 volts is considered rich
the first lambda should pulse between .2 and .8 volts whilst the 2nd lambda should pulse more smoothly, like .3 to .6 volts, note they should pulse around every second or so