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

Started by BahnStormer, May 12, 2026, 16:16

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BahnStormer

May 12, 2026, 16:16 Last Edit: May 12, 2026, 16:25 by 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!

Black 2006: AC & heated leather: 4x Megillian braces, Koni/Tein custom suspension, MTEC+YS+braided brakes, Toyosports manifold, TTE exhaust, Conti PremiumContact2(summer)/ Conti TS860S(winter) / YokoAD08RS (track/summer), Pioneer MVH-390BT + TS-E171ci, FBSW, Robbins mohair hood.

Carolyn

#1
May 12, 2026, 16:24
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.

Perry Byrnes Memorial Award 2016, 2018.  Love this club. 
https://www.mr2roc.org/forum/index.php?topic=63866.0

Gaz mr-s

#4
May 13, 2026, 12:34
Quote from: SurreyMR2 on May 13, 2026, 07:18Literally 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. 

Bravo!  ;)

BahnStormer

#5
May 22, 2026, 20:47
Quote from: Carolyn on May 12, 2026, 16:24Is it a proper type approved cat or a 200 cel 'sports cat'?

Def not 200 cell sports cat.

@headcase said he was shopping around for one of the older spec ones (cheaper), but ended up going for the "approved" 2006 spec one.
Black 2006: AC & heated leather: 4x Megillian braces, Koni/Tein custom suspension, MTEC+YS+braided brakes, Toyosports manifold, TTE exhaust, Conti PremiumContact2(summer)/ Conti TS860S(winter) / YokoAD08RS (track/summer), Pioneer MVH-390BT + TS-E171ci, FBSW, Robbins mohair hood.

shnazzle

#6
May 23, 2026, 10:02
I put a very good quality 200cell cat on and that avoided the P0420 mostly. But for MOT it needed SEVERE ragging and heat before passing. Like, crackling when stationary hot. Granted I had fiddled with stuff so fueling was never perfect.
Eventually I put a spacer in between sensor and cat just to avoid hassle.
But that was a mucho pricey custom cat. A cheaper 200 will 100% lead to P0420, and a cheaper 400-600 cell will 50% lead to P0420. 800 cell and up is your only safe bet to definitely avoid that. Stock is something like 1200cell
...neutiquam erro.

Gaz mr-s

#7
May 23, 2026, 10:39
My partner bought a 2zz-compressor engine with a Rogue-built sports cat exhaust around 6 years ago. It's approx two years older than that.
A dose of Cataclean before the MOT & it breezes through each year.
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