Bit of advise needed.

When i put my 2nd decat pipe(one after the downpipe 03wrx) on i got the expected P0420 cat efficiency code, however also got this one

P0137 02 Sensor Circuit Low Volts(Bank 1 Sensor 2)

i know this refers to the lambda sensor after the down pipe, does this mean the lambda is goosed? Or does this code pop up anyway with the P0420 when you remove the cat after the downpipe?

I have reset it with my code reader, so only time will tell if it pops back on.

Cheers