P0420 code... cat or sensor, I need your knowledge

[Rob350]

Got a p0420 bank 1 or 2 under efficiency threshold, iv got standard cats and y pipe with k1 mid and back, Iv done some reading some people say it could be o2 sensor, others say defo the cat, also seen 1 guy say he had put some redex type stuff in had a blast and it hasn't come back in 6weeks.

Iv read it won't hurt my engine I leave it for a bit.

 

It only comes on when staying at a constant rpm for 5 to 10mins like on a motorway

 

Someone flogging cats for 70 squids on here I'm tempted to buy

 

Any help appreciated chaps

[ZMANALEX]

It is bank 1:

 

Try an O2 simulator to space out the rear sensor from the flow.

 

Might save you a few bob.

[Rob350]

The exhaust was on the car when i bought it 2 months ago and the code only came on last week, so i assumed the spacers were already on