Hi all,

I own a 2017 St-line focus, I have 60k miles and have recently just had my engine light come on.  Upon plugging in my OBD code reader, it tells me code P0420 - catalyst system efficiency below threshold bank 1.  I have changed the lambda sensor pre cat and about a week later had the engine light come back on and has produced the same code.  
 

Before this happened I had changed my oil, oil filter, brakes etc as I was driving up to the Lake District (5 hour journey from where I was) for a few nights.  I returned home and everything was fine, it wasn't until the next day after I had come home that it then popped on.  So having changed my lambda sensor and it still showing there's a problem, does anyone know what the issue may be?  Surely if the cat was failing would it not show as bank 2 on my code reader? 

Thanks for any help! 

3 minutes ago, Bradley_F said:

So having changed my lambda sensor

I know its not very likely but it can happen that either you have a faulty lambda sensor or its not the correct one.

Have you checked the wiring to the sensor look for signs of corrosion on the plug ?

2 hours ago, Bradley_F said:

Surely if the cat was failing would it not show as bank 2 on my code reader? 

No. Your car only has 1 bank (like every 4 cylinder engine I can think of). That code just means the ECU thinks the cat isn't working properly, and gives no reference to a sensor being faulty (although it can be one of the reasons the code is triggered).

P0420 is post cat sensor

More often than not on the earlier 1.0 engines that DTC does truely mean that the cat is faulty. I have changed more cats than O2 sensors on these engines (excluding the new ones 2019+). 

If you could monitor the O2 sensor 2 (post cat) it should give nice a steady reading as the cat is doing its job. If it's a bit erratic then I'd suggest the cat has failed.