Our van doesn't get driven hard, so I'll start by saying I don't know how long it's been like this. It seems to get a little better when it's been running a while, but above 4500, it more or less hits a wall and barely accelerates. Might even start slowing down until I let way off and it finally shifts up.

Potentially relevant info:
Sometime this summer, it started having a harsh downshift when slowing to a stop or for a turn.
Thanksgiving-ish, it started throwing a P0420.
Replaced the plugs (NGK g-power), wires (OEM), O2 sensor (acdelco).
Code persists. I plan on putting in a O2 extender on the 2nd sensor.
It burns about a quart of oil every 2000 miles.

Is the cat plugging up, or what else could it be?

 

First thing I would do is have a air test done on the exhaust, This way you can see if you have an exhaust leak. That will cause a P0420 code, as well as a plugged exhaust. Any shop that has a smoke tester for the Evap systems will have the adapter for the tail pipe.

 

Any way I can test that myself at home? There is no obvious auditory or visual evidence of a leak. I've been under it and checked around.

 

Start it up and hold a gloved hand over the tailpipe. Increased exhaust pressure leaking out a hole anywhere will increase the noise and make it easier to find a leak. Eventually the engine will die.

It does sound exhaust related but I'm thinking it's partially plugged and it's probably the cat that's the culprit. Just from reading more often than not that's the cause of a P0420. http://www.obd-codes.com/p0420

 

In the meantime, does anyone know the difference between Magnaflow's standard and OEM grade cats? Does one flow better than the other, or have higher quality pipe materials?

http://www.magnaflow.com/02catalytic...irectfit=93202

http://www.magnaflow.com/02catalytic...irectfit=49948

 

I replaced the cat with Magnaflow's 49 state unit. The original had deposits forming on the face of the brick. The result is a remarkable increase in power. Mostly just restored power, I'm sure, with a small increase relative to stock given the high flow nature of Magnaflow's products. And the exhaust has some burble now. I'll try to get a pic of what was going on with the old one.

 

Anyone seen buildup like this before?

 

You are burning a quart of oil every 2000 miles - that oil has to go somewhere. Looks pretty typical given what you've said so far.