I was throwing all kinds of exhaust codes, replace all 4 oxygen sensors, did a seafoam induction cleaning and all seemed great. But a few hundred miles later it through a PO420 code. Ideas? My EGR my be faulty (it is rumbling something awful). I know it could be the cat, but I'm not able to shell out $800 for it. Could it be something else? Thanks.

 

"Catalyst system efficiency below threshhold".

If you've just replaced all 4 O2 sensors, it's possible one of them could be bad, perhaps in a way that wouldn't throw a code specific to that sensor as well...

Did you do the cleaning first? or replace the sensors first?

 

I did the cleaning after I believe. But PO420 is not a code for the O2 sensors just a Cat I believe. I could have my shop check the O2 sensor voltages though. My Air Injection Pump seems to be going also (very loud rumble coming from it). Could that be linked? Thanks

 

Doubtful!
What octane fuel are you burning and have you had a head gasket failure recently?

 

I did cleaning after installing O2's. My headgasket give no visible sign it is gone, but I did take it to a LR mechanic who said it needed to be done. I have small oil leaks at oil pan, valve covers, and head. But no strange smoke, water in the oil. It is actually running great and get abt 14 mpg. So I'm not sure if it has "failed" or if it is starting go slowly. Could a faulty SAI pump cause this code? My air valve solenoid is apparently bad. Replacing it today.

 

typically i run 91, but every once in a while i will do 89. Does it make a big difference?

 

The 420/430/1590 code came back after being gone for a while. Thoughts? vacuum leaks? anything else? If it is the cats, what is the best aftermarket you guys recommend? I'm not dropping 3K. Thanks.

 

you can go to an exhaust place near your house and have them weld a cat in place and it won't cost $3k... or you could even buy this http://www.roverparts.com/Parts/WCD105870.cfm and do it yourself.... you need to climb under the vehicle when its cold and tap the cats with a wood block. and see if they rattle then you will know if they are bad.

 

Both cats are starting to fail, how many miles on your truck, when was it last tuned, were wires installed and have you ever had a head gasket issue?