The engine fault light on My sisters 1.25 mkV fiesta has been coming on intermittently for the past few months.

It shows the code P0420 catalyst system effiency below threshold (bank 1)

It has been suggested that the post cat sensor is at fault but I'd be grateful for anyone's thoughts or experiences before I change it.

Many thanks

Martin

Have you just typed that code onto google?

Yes, but nothing conclusive seems to come up... From what I've found out the code is triggered if the O2 levels in the exhaust are the same both up- & down-stream of the cat (indicating its not doing anything) if one of the sensors fails the same apparent problem would show? If I understand things correctly the downstream sensor is heated? (does this suggest that the heater element is prone to failure? I'm guessing here but it sounds plausible ?)

Just wondered if anyone had gone thru a similar thought process & what the outcome was?

Thanks

Hi,

I was having the same issue, but it's now cured for just £7. It's an o2 sensor spacer/cheater, basically positions the sensor away from the exhaust flow to give a weaker reading. I still have a deteriorated cat. but at least the annoying MIL is off.

I spent ages reading into the P0420 code & wrote about it here: