P0420 Cat efficiency

[kevinb70]

1998 3.1 @ 208k

Code P0420 cat efficiency. No issues at all since working on it back then in 2010, except for a 440 gas cap is loose code a time or two.

22k miles since this top end rebuild: http://www.automotiveforums.com/vbul...d.php?t=977985 (heads/intake cleaned at machine shop including valve job, had injectors rebuilt via Mr Injector. Replaced egr, plugs & wires, tstat, magnaflow cat, steel LIM gasket)

Got the code over a week ago, reset SES and did replace downstream O2 - though my thoughts were on that Magnaflow cat.
Been waiting the past few days to see if the code came back once I completed enough drive cycles... sure enough it's back.

Runs smooth and quiet as if it were new.
Coolant temps OK.
Battery 12.6V/charging 14.0V.
Vacuum 19" at idle, returns to 19" quickly after revving engine (cat not blocked)
Fuel pressure: key on - 42psi, idle 38psi, doesn't drop significantly when revving the engine
Not losing any oil (synthetic) or coolant.
Chevron/Texaco 87 gas only.
Air filter is clean and dry.

Cat temps:
Pulling over after driving a 45mph, and immediately checking cat temps: 290F/375F.
Idling for 5 minutes after a drive, pre-cat temp just under 300F, post-cat temp just under 400F (100 degree difference).

Drive like granny and get 24.5mpg mostly 45-55mph and don't get into much rush hour slowdown. Doesn't look like MPG has gone down any in the period it took the first and second SES to light up.

Using Bosch O2 sensors (both up/down).

My thoughts are the efficiency of the (48 state) Magnaflow has declined a little bit and not good enough for the CPU. Or just not getting the temps to fully light up - although I'm sure when driving the temps are quite a bit higher

-Haven't checked plugs - (think they are A/C delco) but I think they'll be clean as I'm not misfiring and still have great mpg. But fouling would suggest the cat is fouled despite the temperature differential is at 100 degrees) However - the O2 sensor I pulled only has very slight white residue.
-Exhaust was reinstalled in 2010 nice and snug... not suspecting an exhaust leak.

Thinking:
-Replace both O2 with non-Bosch sensors (suggestions on brand?)
or
-Replace Magnaflow with new non-Magnaflow (looking at rockauto: Eastern Catalytic's Eco GM, CATCO, BOSAL - favoring the Eco GM)


Thoughts? Ideas?

[Tech II]

Best way to determine if it is the cat, is to plot the pre and post O2 sensors on a scan tool.....if they mirror each other, the cat is bad....

You are correct....non GM cats tend to fail early......

It always amazes me, when I see a car come in, and in place of a huge GM cat, is a minny version of it......just a matter of time....

[jimbob0077]

You did a great job analyzing this problem. However sometimes it’s just cost effective, to take it to someone that has the $10K scan tool that can graph the O2 sensors, and tell you which part to spend the money on.