I am occasionally getting this fault code P0420 (Catalyst system efficiency below threshold bank 1). Car runs fine but every now and again the engine fault light comes on. Having read other forum posts the first thing to do is replace the cat sensors. But my issue is money at the moment, having read other posts I should stick to OEM on this part but at +£200 per sensor I am going to struggle a little. So my plan is to replace one at a time and I am trying to guess which sensor is at fault. This code would suggest to me that it’s the sensor after the cat that’s the issue as this must surely be monitoring ‘cat efficiency’. The only thing that makes me think twice is that about 3 months ago I got another code P0132 (O2 sensor bank 1 sensor 1 high voltage) which is the senor before the cat. So if the sensor before the cat was faulty would I get the P0420 code?
A couple of side questions;
1. How long does the EML stay on after it registers a fault? I thought when you turn off and on the engine the EML would go away unless the fault is still there but this doesn’t seem to be the case. Last time I the EML came on it just went off again while driving.
2. Does the EML mean an MOT fail now? Previous cars have passed with the light on as an advisory but I’m sure somebody has told me that isn’t the case anymore.
A couple of side questions;
1. How long does the EML stay on after it registers a fault? I thought when you turn off and on the engine the EML would go away unless the fault is still there but this doesn’t seem to be the case. Last time I the EML came on it just went off again while driving.
2. Does the EML mean an MOT fail now? Previous cars have passed with the light on as an advisory but I’m sure somebody has told me that isn’t the case anymore.
