Hi, I have a 2008 Z06 427 with 35,000 miles. Recently a code has popped up, P0420 "Catalyst system efficiency below threshold Bank 1". I cleared it and it returned within 100 miles. No other codes or monitors. Seems to be running fine, no other issues. How serious is this? Could it be a sensor or is this pointing to a bad converter? I have only had this car since February, and cant find any history on previous maintenance.
Absolutely love the car! A big step up in performance from my '81!

 

I have 05, and starting to get check engine codes, none was what you got, but saying bad bank for catalyst converter.

I took the car to Muffler shop, they put it up on lift, and with wireless thermometer, came up with left, cat 200 degrees, and the right over 400 degrees, and I had block catalyst converter, then took it two others, and all three all had the same reading, and went with one gave me the best estimate.

Since I had high 70s in milage, I replaced 02 sensors all four.

PS Not one Muffler charged me to check the car out there out to make a buck, and did for free, they all do it.

 

I found this list on C6 Tech Performance forum and member Dano523, and your code is on the list.
This might help you!

Rear OS sensor are just monitoring the cats only. They have nothing to do on how the engine runs, so yes, then can be turned off to they are not reporting errors in the tune.

So in either HP tuner, or EFI live, go into Engine DIAG, and uncheck SES enable boxes and change the last line to no error reported in the list below, then flash the tune back into the ECM with the error reporting changes.

P0036
P0054
P0056
P0060
P0137
P0138
P0140
P0141
P0157
P0158
P0160
P0161
P0420
P0430
P2270
P2271
P2272
P2273


Hence we are just turning off the rear bank 1 and 2 O2 sensors/not allowing the Ecm to send errors on what they are reporting.

 

Drove an '08 Z06 home from dealer 12 years ago, still have this beauty.
Last Spring mine threw a P420, dealer said RH O2 sensor AND catalytic converter shot.
OK says I, replace both...then it took a twist, OEM CCs were backordered w/ no known time it'd be available.
Not good, was already overdue getting it registered so SA says we'll put on an aftermarket CC.
Less $ than OEM and, they'd warrant it as-if OEM. Great. Do it.

Maybe w/in 500 miles notice LS7's 'bellowing' on decal, 'sour', beautiful tuned exhaust note gone.
Isolated annoying bellow from RH muffler, thought. WTF.
Embarrassed to admit what I spent trying to get hat sweet LS7 sound back before I figured out it could only be one thing: That AM CC.
Bought a nearly new CC from eBay at half GM's retail, had it installed & voila', back to normal.
I swore I'd keep this Corvette forever, it's that special; however, had I not got it back I'd seriously have considered flipping.
Those I spoke w/ re this recommended upgrading exhaust system, headers etc, I wanted to remain stock.
Lesson is if CC is replaced, insist it's OEM.

BTW since battery had been disconnected for work, waited ~350 mi before damned registers would reset to pass DMV.
~350 mi I was an outlaw.
So keep that in mind whenever power's interrupted, not only windows need re-indexing.
Smog registers take a varying # of miles before they're ready to pass an inspection.