Reoccuring P0420 , no funny idle ; but poor fuel economy..

[robert208]

I have heard of pinholes at y-pipes tripping the ecu on some 350's , could be clogged cats ; but the car idles very quietly and smooth leading me to think otherwise as there is an equal amount of vapor upon cold starts.. been clearing the code for a month and would love to hear what some of you guys think ; my fuel economy has slowly been dropping since I purchased the car from 20 to just under 17mpg and i drive like a grandpa maybe 3 times a week... ? I'm going to start with the bank 1 sensor 1 02 sensor and likely spark plugs and and a fuel system cleaning... 

[steve2006]

Hi Robert and welcome to the LOC.

You really need to check the Live Data from the O2 sensor to confirm if this is the cause but it is generally the main suspect with this code, you don’t mention how many miles/KMs the car has done or service history?

If you do replace the sensor go for a genuine or branded replacement ( normally Denso).

[Big Jim]

It's the sensor after the cat that is giving you this reading, you can cheat by adding an extender in for the sensor. That will stop the code appearing   https://www.ebay.co.uk/itm/255190880700

You will probably need a special socket to remove the O2 sensor  https://www.ebay.co.uk/itm/353735256608

[Texas]

Top tip, when using these ‘split’ type sockets to undo ‘F tight’ sensors in aged exhaust systems put a  suitable size Jubilee clip around the socket to stop it even thinking of splaying and not staying in total contact with the sensors Hex contact points.