Has anyone had these codes before and what did it require to fix? OBD2 system codes on a 1996 xplorer sport. Has new plugs,wires fuel filter oxygen sensor on bank one updraft, new airfilter,and Iac valve. engine is 4.0 pushrod. also new pcv valve. All this completed last week. got the code yesterday and only after changing o2 senser on bank 1. Now that the o2 is working I guess it's picking up something wrong. The code definition is Catalyst system efficiency below threshold (bank 1), same deff for both codes. why two codes that mean the same thing? whats different about them? Thanks for any insite to this problem.

Philip

 

In my book the code po420 is "Catalyst system efficiency below threshold(bank 1)". And po421 is "Warm up catalyst efficiency below threshold(bank 1)". 1 of them is for warm up. Hope this helps.

 

Thanks gcozmo. I took it to auto zone and had them read the codes and they did'nt
give me a code per say they just said that the "Catalyst system efficiency below threshhold (bank 1) was what caused the cel to come on.I bought a Haynes repair manual and looked in there and those were the codes as the manual had them. I have no idea what to do. Gas milage is very poor do to the fact that when a code sets it goes back to open loop, per manual. Thanks again

 

anybody got any ideas???? need help

 

I think that it is the 02 sensor downstream of the catalytic converter. According to my Haynes manual (1991-2000) in section 6-20 under #11 Oxygen sensor it describes how that sensor operates. I have a 96 and tried changing 1 sensor with no luck and decided to change all 3. Sorry I can't be more help.