P0420

[Ray Brown]

My CEL is on and it is coming up with code P0420 which states catalyst system efficiency below threshold conditions. Engine running at steady state highway conditions for 5 minutes in closed loop, then the PCM detected that the rear HO2S switch rate was too high. Can anyone help me with this? Searching online it says many of things. Some say the rear o2 sensor some say the cat. It is also studdering and shaking sometimes. It did the same thing before when I had ignition coils go at different times. I replaced coils number 1,2 and 8 already. Any help with this matter would be great since I just had to have a front wheel hub replaced this weekend. Thanks ray.



2007 ford explorer sport trac v8

 

[scott degroot]

Before you can properly diagnose a catalytic converter failure the engine has to be running well. You need to get your misfire (studdering and shaking) fixed before you try to diagnose this. A misfire left unchecked WILL cause the catalytic converter to prematurely go bad though...I hope your not too late.

 

[Ray Brown]

Thanks for your help. I have already fixed the misfires. The internet is saying it is the o2 sensor after the cat. Any idea if that is correct?

 

[Vic Sorlie]

Found link below that has more info than what you say. What I see is that it could be either sensor, and looks to me like you either replace both or take it to someone who knows how to properly diagnose. Let us know what you did to fix.

 

[scott degroot]

FWIW I have never fixed a catalytic converter code by replacing the after cat O2 sensor. You will need a scan tool to diagnose the problem. Do you have access to a scan tool?