Let's talk catalyst efficiency and diagnostic theory

2006 Buick LaCrosse 3.8 Cali emissions, recurring p0420

I should say straight away, I found out as the car was leaving my driveway today that the cat was replaced 2 years ago, aftermarket cheapy.

As I've always been taught and have practiced, the way to test for a failing converter is to monitor the O2's during a cruising decel event, and judge the separation of the sensors seeing the lean condition caused by decel fuel cutoff.

This capture is a couple seconds of WOT, follow by complete liftoff of the pedal. You can see the upstream sensor nearly mirrored the throttle pid, but downstream stays locked. That's a brand new Delco O2, btw.

I noticed that the upstream sensor did not act like there was a decel fuel cutoff. It returns to stoic almost immediately, if it ever left at all.

So I decided to do a run where I'm monitoring commanded pulse width, and sure enough the cutoff does not happen. Rear O2 movement doesn't really happen either. You can drive it a bit further rich when you floor it, but it doesn't ever really get under 500mv.

Also, why did the upstream go rich on this liftoff capture?

Assuming we can trust the O2 data, is it a sign of a bad cat that downstream sits flatlined in the rich side forever and barely responds to me inducing rich/lean conditions?

I dunno if I have the answers? :silly: But I have some questions!

This capture is a couple seconds of WOT, follow by complete liftoff of the pedal. You can see the upstream sensor nearly mirrored the throttle pid, but downstream stays locked. That's a brand new Delco O2, btw.

...

So I decided to do a run where I'm monitoring commanded pulse width, and sure enough the cutoff does not happen. Rear O2 movement doesn't really happen either. You can drive it a bit further rich when you floor it, but it doesn't ever really get under 500mv.

I'd be interested to know what the loop status was during these events. Theoretically yes, it should have gone into fuel cut decel. BUT, the PCM will do what it wants. :lol: Both GM's and Ford's have thrown me curve balls when I fully expect them to fuel cut, and they just keep on working away in closed loop.

Also, why did the upstream go rich on this liftoff capture?

Not a clue. :blush: Again, what was the loop status? If closed, what were the fuel trims doing?

Assuming we can trust the O2 data, is it a sign of a bad cat that downstream sits flatlined in the rich side forever and barely responds to me inducing rich/lean conditions?

IMO, that seems more indicative of a lazy sensor. OR a screwy aftermarket cat.

Have you observed the PCM actually running the catalyst efficiency test yet? It's a cool thing to see taking place, once you know what to look for.

It appears to be that the Cat is unable to store enough oxygen.
I would not condemn a cat from only using live data though. 5 gas or other testing. What is mode 6 saying? I would drive those sensors with propane and a vacuum leak to see how the sensor reacts and a test light to drive it high and low to see how the PCM reacts.

Which O2 was replaced upstream or downstream ?
The lack of complete fuel cut-off is puzzling, I never seen that before.
What is the CTS showing and what are the fuel trims like ?
Even though it it is an aftermarket cat if it is a to Cali specs it should have a lot of storage capacity, I would give it a big vacuum leak for a prolonged time and see what happens on the rear o2.

Agree with Andy,

Which 02 was replaced and what are your ST&LT fuel trims showing...

O2's on a good CAT would show S1 osculating and S2 fixed at high voltage. With higher RPM S1 should osculate faster and S2 should have low voltage.

Add propane, S1&S2 should fix rich (high voltage). Remove propane, should have about a 4 to 6 sec delay of S2 going to low voltage before S1. If less than the 4 sec CAT is going bad.

Have to have CAT's HOT to do test. All this is dependent on 02 sensors working properly.