I thought I'd cleared up the ECU problems on my XJ8 3.2l 2001 model (last lot were PO128 - dodgy temp sensor connection, and P1647 - upstream Lambda sensor).
I had emissions checked immediately after resolving these and they were lowest they have been for years.


Everything running nicely for a couple of weeks with improved performance and consumption then today up pops another ECU warning, this time PO420.


Engine has been running well and this doesn't seem to have affected it, but as it's only just happened can't yet be sure.


I understand PO420 indicates Catalyst Efficiency Below Threshold for Bank 1, but before I start spending more money chasing components I thought it best to throw myself on the mercy of you forumites and seek guidance.


How likely is it to be the cat rather than the downstream O2 sensor, or perhaps an exhaust leak or ??? and what is the best way of identifying the cause?


I assume that running rich due to low temp detection and lambda problems could have caused a deposit build up in either cat or sensor.
Could this be marginal, is there an easy way to test these, and is there any realistic way of cleaning them without need to replace?


As these are not cheap items I am reluctant to keep throwing money at the problem on a hit and miss basis so would appreciate all advice and guidance.
John