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

I'm not saying it's not down to a faulty ECU.  The only way to find out is to install another one.  You'll need the key that goes with it.

It's also possible that the VVT hub is not performing as it should... That would still, probably, manifest as a poorly functioning OCV.

Have you checked the cam position sensor?  It also could have a bad earth.

Nice little write up from Sevenoaks District MC on Facebook of the Brands Hatch Autosolo and some nice piccies too: https://www.facebook.com/100063456275903/posts/pfbid02UesCxPPhTvLy61wobbZJas5yQXM1qmDBbG5mnmxgiYMtd2BmtGHREgFvSMr3Dbtvl/

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.

Also checked live data for a few reading and all seem ok

Begining to think ECM as that's what sends the signal crank sensor all good only code Is P1656 that won't clear

I do not know the size of your sub but I have an amp (for front speakers) and sub under the dash on the driver side, there is a lot of room there compared to passenger side

Thanks I will check it out

@inigopete

I used piggy back fuses to connect a sat nav and a dash cam.
There is a fuse box in the A pillar on the passenger side , accessible with the door open.
From there it's quite straightforward to get into the car.
You can select a suitable power fuse to piggyback onto.
That was a few years past but still available and some are reviewed here:

https://toproducts.co.uk/fuse-piggyback

Other suppliers are available.

I have a Kenwood sub behind my seat. As the sub power supply had a fuse with less than the cigarette lighter was fused at I used this power as I do not use it for anything else. Been fine for 2 years.