My 4.0 (05 B4000) is throwing the P0420 code. Catalyst efficiency low-bank one. (right). Tester says it's either:

O2 sensor the catalytic converters ability to store oxygen

Catalytic converter defective (failure possibly)

Engine misfire or running condition

Large vacuum leak

Engine oil leakage int o exhaust valve guide, piston rings.


What is my best line of diagnosis?

How do you check for a vacuum leak?
Should I replace the O2 sensors (upstream and or downstream?)
Buy a new cat? (don't want to if possible)

I have no odd smoke or smell from the exhaust. The motor is running fine. I don't see or feel any problems other than the CEL is on.

What to do?

 

An OBD II V6(4.0l) will have two "upstream" O2 sensors, one on each exhaust bank(manifold).

Then it will have one or two "downstream" O2 sensors depending on the exhaust system, single exhaust or dual exhaust, these are located after the Cat converter(s).

On a reader they are listed as
Bank 1 O2 #1, Bank 1 O2 #2(passenger side)
Bank 2 O2 #1, Bank 2 O2 #2(drivers side)

On a single exhaust system there would be no Bank 2 O2 #2

The #1 sensors are used by the computer to tune the fuel mix, lean/rich, these are the "upstream" sensors
The #2 sensor(s) are used to check if Catalytic convert(s) is working, these are the "downstream" sensors

Code P0420 means #2 downstream sensor is reading high carbon levels.
So either the O2 sensor is bad
Or cat converter is not working as well any more
Or engine is running way to rich<< this would be a long shot since #1 sensor "rich code" would be on as well.

You could swap any other #1 O2 sensor with the #2 sensor, or replace that O2 sensor, BUT............the #2 O2 sensors rarely fail, not never just rarely, these sensors get nice and clean exhaust after the cat converter, so last pretty much the life of the vehicle.
The #1 sensors often need to be replaced at 100k, but that's usually only if you have had engine trouble in the past with lean or rich running over long periods.

If you can live scan the O2 sensors you might get more info, #2 sensors shouldn't jump around like #1 sensors will, they should stay at a low number.

And it could be a #1 sensor because the #1 level and the #2 level is compared, that's how computer determines if Cat is cleaning up carbons from exhaust, but that is a long shot.

 

Since I don't really have a way to check the cats, should I replace the downstream 02 sensor first? (single exhaust).

I guess I will have to take it to a shop to get the cat checked.

I have 80,000 and bought the truck used so I have no idea if there were past engine problems. I am getting the same mileage as about everyone else, about 16

Thanks for your reply RonD.

 

Yes, or just swap it with an upstream O2.

Shouldn't have a Cat issue at 80k, or an O2 sensor issue for that matter

 

OK, I took it to a shop for advance diagnostics. Mechanic friend said that the O2 sensors are electronically working fine. The rear cat is OK but the small cats on each side of the exhaust manifolds are clogged. The y pipe which is where the cats are (exhaust manifold) is a 1300 dealer replacement. It is not effecting performance. For 1300 I can drive a long time with the CEL on.

What are my options for replacing the y pipe (with built in cats) other than new factory ones? Do aftermarket headers have the electronic hookups that will keep the CEL off?

Just looking for options. Thanks in advance

 

If you knock the cats out of the y tube do you still get the CEL? I suppose it will fail the emissions testing even if there is a downstream cat?

We don't have emissions testing at present but have in the past. Wouldn't be surprised if they start it again for additional tax collecting...this is Illinois.

 

You get the CEL from the comparison values of the upsteam O2 levels and the downstream O2 level so with bad cats or without cats you would still get the CEL

I would hit the wrecking yards for replacements, cats are not electronic devices so while they can chemically wear out or rust out they are still fine to use if they "look" good.
If the current Cats are starting to fall apart, clogging up, then it will start to effect top end performance and then steadily decrease performance at lower RPMs.

Seems way too early for cats to fail like that, since you don't have a history on the truck my bet would be a long term rich condition was present, which caused the premature failure, if truck runs fine now then condition was fixed but damage was done for premature failure of cats.

You can also see this on "low miler" vehicles as well, short trips without 20 minute drive time causes condensation build up in oil and exhaust systems, this rusts the exhaust from the inside out, cats have the highest surface area so get the most rusting.