P0420 Error code diagnosis help needed

[Paul-H]

Hi all

Had this problem with my 2003 1.4 Fiesta for some time but now getting round to try and fix it

for the last 40k miles every 500 miles or so the EML pops on with an error code P0420 Catalyst System Efficiency Below Threshold (Bank 1)

So how do I find what is the route to the problem, I assume either one or both o2 sensors or the cat itself could be at fault, all quite pricy on a car this age so don't want to go down the try and see route.

How do I test the sensors to double check they are at fault.

I have monitored the sensor voltages whilst driving assuming that this would indicate if one or both is not working or if both have the same reading I understand that would indicate the CAT has failed.

I used Torque to log the data but can only get it to send screen captures not the whole logg

Looking at the three attached captures do they indicate anything obvious

Thanks for any help resolving this

P View attachment 321 View attachment 322 View attachment 323 aul

 

[Mike]

With that code you probably have a bad Catalytic Converter.

 

[Paul-H]

Thanks for the reply

If it was a bad CAT would the sensors be giving very similar readings as they are reading the same gas in and out of the CAT my sensor readings are all over the place but different.

Should they not be at least similar if the CAT is not removing enough particulates.

Paul

 

[Mike]

The easiest route would probably be to replace the down stream o2 sensor first. All the second o2 sensor does is measure how effective the catalytic converter works. If it detects an inefficient catalyst then the light CEL appears.

 

[Paul-H]

Anyone know if the two sensors are the same or different

One dealer has told me they are the same but others list two different part numbers

Would be good if they are the same as if changing one does not fix it I could try it in place of the other without extra expense

Thanks

Paul

 

[rooSTer]

The two sensors are different on my 2014. I would assume its due to the cable length. Might be the same for the older models.

Anyone know if the two sensors are the same or different

One dealer has told me they are the same but others list two different part numbers

Would be good if they are the same as if changing one does not fix it I could try it in place of the other without extra expense

Thanks

Paul

 

[Steve@Tasca]

The sensors are different, I'd offer to help with parts/part numbers but we never got the 1.4 here in the States.

-Steve