Classic P0420 "Bank 1" error - anything to do before replacing the O2 sensor(s)?

[BahnStormer]

I've been trawling the threads (e.g. https://www.mr2roc.org/index.php?topic=67682.75) to see if there is something the same as my scenario, but nothing is jumping out

My car had a brand new cat fitted about 4 months ago and has started to flash up the EML.

The error code it is returning is P0420 "Bank 1".

If I reset it using Torque Pro, it is fine for a week or two (multiple 3-5 mile trips almost every day).

The cat was fitted by Pat Chambers, so I am assuming that installation isn't a problem.

O2 sensors are old, so it's a fair guess that need replacing.

Questions:

1) is there anything I should be checking as per the thread above before replacing sensors?

(that thread seems to be all about a failing cat, but if mine is brand new, I'm not sure if I'll get any useful info from a failing sensor?)

2) With that error is it just the lower one or do I need to do both the manifold sensors too?

3) Denso part numbers?

Googling around I think they are these two: Denso 234-4624 and 234-4603

Hoping to get away with just the lower one at this stage **fingers crossed**

Note I've got the TTE exhaust and ToyoSport manifold, but I've had these two combined without issue for most of the last ~10 years!

[Carolyn]

Check for leaks at the manifold to cat joint.

Is it a proper type approved cat or a 200 cel 'sports cat'?

The post cat sensor will have been subjected to a bit of a hammering removing it from the old cat and these sensors are quite delicate, but I would check for leaks before spending the money.

The other two sensors should be fine.

[Gaz mr-s]

This is your likliest cheapest option. https://www.ebay.co.uk/itm/266231049216?fits=Car+Make%3AToyota&_skw=Denso+universal+o2+sensor+toyota&epid=18039424586&itmmeta=01KRF0BG53J0QV65NYMTPHTF36&hash=item3dfc9b6c00:g:HV0AAMXQMmJRS1E0&itmprp=enc%3AAQALAAABAGfYFPkwiKCW4ZNSs2u11xDtvEwGHAsRT3cutmY3f7wowoqzCW%2BUw%2F1yLB9mYpM1P6MSof4lAfBlu8IgWuZcmTdEpx5%2B7QUFG6zywX6km2RbOL%2F2RZFNiAn3cP1Hwi3ZIDnXcunJT%2FYdf%2FtmwiSCudFZXaIuAJwNx0Xsn7r3rAWFN1VRGyxt2J5AEk9ywkjXRB0hzRMrWxrWmEywmXf3weJBfavwkX%2BbvCGYnYJ9afvOCjO7rNxxHXzzbvF5lfaYRaPXBqBzi23e4afdDNcKivTuByiXRbMTLUg%2FcvBN8Zfhu1OHBr%2FqQYfWoHiR%2B9ZzliGdq3hGOiS9HIP659brIgM%3D%7Ctkp%3ABk9SR_iCruDDZw

Check ECP though, sometimes they have good prices on them.
Bear in mind the sensors are all the same, you just need 4-wire, thread fitting, Toyota plug & a long enough cable.
I've found a fe 'bargains' on ebay over the years.

[SurreyMR2]

This is your likliest cheapest option. https://www.ebay.co.uk/itm/266231049216?fits=Car+Make%3AToyota&_skw=Denso+universal+o2+sensor+toyota&epid=18039424586&itmmeta=01KRF0BG53J0QV65NYMTPHTF36&hash=item3dfc9b6c00:g:HV0AAMXQMmJRS1E0&itmprp=enc%3AAQALAAABAGfYFPkwiKCW4ZNSs2u11xDtvEwGHAsRT3cutmY3f7wowoqzCW%2BUw%2F1yLB9mYpM1P6MSof4lAfBlu8IgWuZcmTdEpx5%2B7QUFG6zywX6km2RbOL%2F2RZFNiAn3cP1Hwi3ZIDnXcunJT%2FYdf%2FtmwiSCudFZXaIuAJwNx0Xsn7r3rAWFN1VRGyxt2J5AEk9ywkjXRB0hzRMrWxrWmEywmXf3weJBfavwkX%2BbvCGYnYJ9afvOCjO7rNxxHXzzbvF5lfaYRaPXBqBzi23e4afdDNcKivTuByiXRbMTLUg%2FcvBN8Zfhu1OHBr%2FqQYfWoHiR%2B9ZzliGdq3hGOiS9HIP659brIgM%3D%7Ctkp%3ABk9SR_iCruDDZw

Check ECP though, sometimes they have good prices on them.
Bear in mind the sensors are all the same, you just need 4-wire, thread fitting, Toyota plug & a long enough cable.
I've found a fe 'bargains' on ebay over the years.

Literally just bought a Celica item with a long lead as fitting a zero manifold so need longer lead. Found new old stock Denso item on eBay this week at great price.