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