Ford B Max 1l ecoboost

My car has been showing an intermittent P0420 error code for a while now. This indicates poor performance of lambda sensors, catalytic converter or both. The sensors are due for change around 160k km which is about 100k miles. I've only done 75k but given the message I thought Id better investigate. The sensor have never been changed to my knowledge so these seem like the best place to start. I bought lambda sensors from Bosch as both pre-cat ( 0 258 017 290 ) and post-cat ( 258 986 602)  sensor versions. Luckily access to the sensors is pretty straight forward from under the bonnet (pre cat) and from Under the car (raise on ramps) post cat.  I used "shock and unlock" to ease the sensors and this worked well.  Both sensors could be loosened with lambda electrode sockets and these worked well. I changed the post cat sensor first, it was badly soiled with carbon and had clearly had a hard life. The new sensor simply screwed in. The guidance is to treat this like a sparking ...