Catalytic Converter CEL

This comes from comparison of the two O{2} sensor outputs. Normally the engine cycles lean and rich, around the perfect fuel-air mixture. Apparently this is needed to keep the NO{x} and CO catalysts working properly and the engine controller uses this to trim the mixture. Possible errors include failed or failing O{2} sensors as well as a catalytic converter that has been 'poisoned' by bad gas or even ... excessive rich or lean mixture. The ambiguity group consists of:

• Gas leakage in exhaust system - extra air makes it too lean
• Open or short in heated O{2} sensor
• O{2} sensor
• Three-way catalytic converter

Do you have a scanner that is able to read out the O{2} sensor values? The reason is a failed O{2} sensor is a lot easier to replace than the catalytic converter. Swapping them if you have the ability to monitor the signal would tell if the problem went with the part.

Failure in this could lead the car to not trimming for optimum fuel mixture. Definitely, bad gas additives could have poisoned the catalytic converter. But there is one not often discussed. Engine oil in the past or oil additives could contain minerals that poison catalytic converters. It is not common but it happens.

GOOD LUCK!
Bob Wilson

 

If I have the downstream O2 sensor reading much the same as the upstream O2 sensor (very high from what I've read, downstream stays around 0.75 to 0.9 v when being used), is this the tell-tale sign for cat replacement? Or is it still worth it to see if an O2 sensor might be the issue?

Along with the 0420 code, I'm also getting a 0440, 0441, 0446 code. Is there any way these might be related? i.e. an evap issue is the main cause. In doing some research I've read that topping off the gas tank is not good and can cause damage around that area; unfortunately this is a habit I've always done so I feel like that might be a factor.

Are these definitely two separate issues or is there any way they may be related?

Thanks!