my '98 civic has this code and is getting about 23 mpg (city driving mostly) which seems pretty bad. It runs fine, no idle or acceleration issues just really bad mpg. I've only had the car a couple months and I don't think the previous owner did much maintenance on it but it does only have 144,000 miles. Do you think it's my o2 sensor or the cat? I'm also planning on changing out the air/fuel filters and changing the spark plugs and wires to help with the gas mileage I'm just hoping it's not the stupidly expensive cat... anyone had this issue and have any thoughts?

 

My 98 civic just started same code on the weekend 234,000 km
Hope we get some feedback.

 

I believe a PO420 code has to do with the cat not working correct, when the reading between the front 02 sensor & the rear 02 sensor are different. Have you checked your cat? Do you have an aftermarket exhaust?

 

I had same code, unbolted cat & it had a big hole in it. replaced 9 months ago & all good.