Fault code P0420, further diagnosis

[fathands]

Hello,

I’ve had the following code pop up on my diagnostics for a while now. 
looking at it’s description it is either:

Possible Causes

• Leakage in Intake and/or Exhaust System
• Catalyst faulty
• Oxygen Sensor(s) faulty
• Oxygen Sensor(s) Control fault

i don’t really want to just start throwing money at it by replacing a part at a time, is there any way to diagnose further?

Which o2 sensor may be faulty, how do I test them and what readings are out of tolerance?

I assume if the o2 sensors are testing ok, then the catalyst is likely to be the issue?

Any help would be greatly appreciated. 
 

Thanks

Andrew

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