Both catalytic converters go at the same time?

[RichardSEL]

Last year had the infamous p0420code come up that was cleared by mechanic (£30)
Within a few hundred miles it returned had it cleared again (£30) with advice to use a course of Cata-Clean.
I did two courses with the correct low tank of fuel lkevel -- that's lasted about 500 mile

So having wasted £60 instead of taking the earlier offered good advice which should've gone toward the iSoft i930 shelled out the £90 for one --
Now both the "Bank 1 below threshold" and the "Bank 2..." with accompanying MIL came on after about 200 mile.

Is it usual that both cats go at the same time? Or do the sensors, two per cat, go at the same time-ish?

It's an '01MY 3.2L with 123k up -- just had the plugs changed (but not for Iridium ones, just the correct NGKs) and the air filter. In mixed mode driving it returns about 22mpg. There's no misfiring and it starts on the button. No performance difference noted either: feisty and willing

Don't have a 'scope nowadays so can't see (and waistband wouldn't allow me to get to) the sensor's waveforms

If it's both cats to change out, should all four sensors be changed too?

[RichardSEL]

Just seen in:
viewtopic.php?f=86&t=6902
that failure of pre-Cat sensors do throw up a different code. Will assume they're alright as no codes seen for sensor failure

Still would welcome any opinion on whether both Cats would fail at the same time?
Getting fault code for both banks when it comes on -- which is irrespective of whether I'm driving it hard or not

This site:
https://www.onlineautomotive.co.uk/Jagu ... rters.aspx
offers three Cats at £85, £91, and £145 ea inc. What's the difference?