Don,t condemn my Cat , just yet

P0420 for sure. But but but, do have small exhaust leak upstream of the cats. Motor temp < 175 always. Truck runs great. Watched SD videos forever. Help me not condemn Cat. 02 sensors wave forms suggest no an 02 sensor problem. Help!

Well, I would test upstream and downstream O2 sensors, via propane and a created vacuum leak. SD has numerous videos on this. In a nutshell, added propane to the intake should make all O2 sensors go rich. A created vacuum leak should cause all O2s to go lean.

Ensure they respond quickly and aren't lazy as well.

If you know you have an air leak before cat, I would suggest fixing that quickly. That could burn a valve which would suck.

Via a scan tool, I also suggest watching all the O2 sensors as the CAT(s) light off. The rear O2 sensors should all of a sudden behave differently than the upstream O2 sensors, flatten out. The upstream O2 sensors should continue to oscillate quickly while the downstream O2 sensor should flatten, assuming you're just idling.

Also, using a five gas analyzer will end the research quickly.

P0420 is flagged up when the engine computer compares the downstream sensor for each bank with the upstream sensor shortly after the engine reaches running temperature. When up to temperature at idle a conventional narrow band upstream sensor should a constant hi-lo-hi-lo-hil-lo roller coaster pattern between 0.9v (rich) and 0.1v (lean) about once per second.
The engine computer will flag up a P0420 if it either detects a lack of changes from the the downstream sensor or it detects the downstream sensor is following the upstream sensors hi-lo-hi-lo-hi-lo pattern too closely.
To start with you should seal the exhaust leak before the cat.

So the next thing is to check is if the upstream sensor B1S1 & B2S1 are following this patterrn.







There are number of ways you can drive the upstream rich I usually do it by either introducing propane to the intake or simply fluttering the throttle .

Andy.MacFadyen wrote: There are number of ways you can drive the upstream rich I usually do it by either introducing propane to the intake or simply fluttering the throttle .[/attachment]

Very helpful description and images, thank you - was that meant to read "drive the DOWNstream rich"?
Also how does this happen, does it mean that the CAT simply can't handle the extra load of fluttering the throttle?

The rear O2 sensor 'settles down' because the catalytic converter is 'cleaning' up the exhaust gas by causing the gas to oxidize into co2 and h20.

The catalytic converter in the simplest terms is like an oven containing metals which promote reactions to occur. So if is working at idle, the rear O2 should show that it is working, by the rear/downstream O2 sensor indicating appropriate exhaust gas reactions are occurring, leveling out as compared to the front/upstream O2.

There are certain driving situations when the CAT cannot work quick enough, like quick snaps and the like. This would make the downstream O2 look more like the upstream O2.

I'm waving my hands in the air, meaning I super simplified what is going on and leaving out tons of details. But, I thought this would get you going. There are tons of information online of how a CAT works, if you want more details.

Thanks guys I just remembered I posted here, but never got notifications of responses. Any ways, yes those pictures are great. I think I saw them in a SD video. Bank 1 02 sensor 2 (post cat) does mirror the bank 1 upstream 02. I did get a thermostat to get the motor, exhaust, cats ,and 02s to run much hotter than current ECT at <175. It was just a last idea type thing, and I'm not throwing any more money at it. I may do the propane test as that is all I have access to, and possible swap some of my 02s swap the 2 downstreams is really all I need to do to confirm. Just don't wanna give up yet! Darn truck actually runs great after I did a motor swap. Detonation caused a piston to blow a hole. That is the same bank the cat is on and most likely related. Lesson learned: Flashing CEL? Drive to a safe place - turn off motor. First time I even worked on an OBDII Vehicle. Do not even qualify as a backyard mechanic. Thanks guys....

GeekDIYMechanic wrote: If you know you have an air leak before cat, I would suggest fixing that quickly. That could burn a valve which would suck.

How does this happen please?

I forgot to say I will also try to create a lean condition, hey its free. Also, the exhaust leak (heard only thru a cut off garden hose) is right at the point where Y-pipes meet up with the exhaust manifolds. I know it is there because I heard it, and have tried 3 times to get those aligned and bolted up TIGHT. Not easy, but must give it one more shot, again, a free option.

Desmond I do not know why an upstream exhaust leak will burn up a valve. I suspect to much hot air, or just to much air is backfilling into air intake. I really don't know.

Edit: From google - A leak in the exhaust manifold can burn the exhaust valves, which compromises their seals over the cylinders, and impacts the performance of the pistons, hence the engine. If the manifold is cracked or has a leak, the back pressure in the exhaust system will be incorrect, which reduces engine power.

If when warmed up the upstream 02 signal looks normal and downstream sensor signal is closely following the upstream it is 99% certain that the cat is history.
As there has previously been a major engine blow up it is more than possible the cat has been shattered.