P0420 Fault Code

Hi,

I have this fault code(p0420) on my t4 camper(I still have a gti just thought I may get good advice on here)
It says on the OBD reader, Catalyst System Efficiency below threshold(bank1)
To me me this either means the cat or the pre or post lambda sensor is at fault. However the Cat was changed 5000 miles ago and is only 18 months old, so I am leaning towards one of the sensors. My problem is I don't want to change parts until I fix the problem(could get expensive). I appreciate the CAT could be at fault even though it was a recent change.

What would be the best point to start? It is a 2.5 Petrol running LPG.

Any help would be appreciated

 

Ever had a fault code saying fuel trim/system running rich?

 

Tommy- nope never had that code. I tried a reputable Cat cleaner but didn't work.

What leads do you scope on the sensors? I take it those images are what you should get on the scope if the sensors are serviceable?

 

Cool,Just asked as I got the same cat code after running with the system rich one. too much fuel can ruin a cat converter,hence the question.

 

Yeah I am hoping its not the Cat but you never know! Guess its time to blag the Oscilloscope from work!