Last month, I had a MIL light on, but the 'Vette seems to be running & driving fine. I had an emissions test and it failed. They were telling me that it put a code PO420, meaning there was a problem w/ the cat on the driver side.
So, I make an appt. w/ the dealer and tell & show them the paperwork and they find that the cat needs to be replaced. So I have the work done, and MIL light is gone but pops up again on way to get emissions re-tested, so I didn't even bother, drove it straight home.
A week later, I decide to try disconnecting battery & then hooking it back up, drove to the emissions testing place and they told me the Corvette was rejected, meaning the OBD system is not ready and they told me the cat has to be broken in thru a few hot/cold cycles ( something the dealer did NOT tell me) so I left there, drove around for over an hour and MIL light comes back on again!

So, will this go away eventually after more driving cycles ? I have no damn idea.

 

Same happened here couple years back.
This'll happen when power's disconnected also, BTW.

Takes miles, sometimes 50, sometimes >100 for emissions to reset.
Keep driving, eventually registers will be 'ready'.
Some used an OBD2 scanner to see when it's ready.

 

Some Cats do have a break in period. Hopefully the dealer did this for you. If you are still getting the P0420, there is a good chance the cat was not the problem to begin with.

 

There are two things going on.

Whenever you clear codes, whether with a scan tool or by disconnecting the battery, the internal readiness checks are cleared, too. The checks are performed automatically during a "drive cycle." Until the drive cycle is complete, an emissions test cannot be performed.

Being in the "not ready" state will not illuminate the Check Engine light. If your Check Engine light is on, that's a separate problem. A DTC will be stored.

The next step is to use a scan tool to retrieve the DTC for diagnosis.

Stop disconnecting the battery. This just resets the readiness checks and throws away the DTC you need to retrieve.

 

My first question is WHY did you take the car to a dealership for a diagnosis ??…that is the LAST place to take it !!…dealerships are pretty much clueless when it comes to diagnosing…they just replace parts until they hopefully fix it and you wind up with a big bill…there are Techs that just diagnose cars if you didn’t know that…they can diagnose a Honda or your Corvette…they just have a specific set of skills that other techs don’t have which are their specialized equipment and knowledge…now do you know there is a recall (specific 08's and 09's) on your cats (below that was declined by the PO of my 08) so I hope you didn’t have to pay for them if yours was one of them…they are also supposed to reprogram the ECM…was that done ??? …P0420/0430 is an “effect” and not a cause…the dealership should have done an “oxygen storage test” before replacing the bank 1 cat but I’m certain they never even heard of that test…cats usually just don’t go bad…misfires, damage by coolant or blowby gases can nuke them...also an exhaust leak between the upstream and downstream O2 sensors can cause this DTC to set…and don’t disconnect the battery because that IM Monitor will not set…the dealer should NOT have given you the car back until that Monitor was set…according to Service Info (below) it’s says it MAY take up to 100 miles for that DTC to clear…that TSB probably doesn’t refer to your situation now but I’m assuming the ground for the coil packs (G107) is causing misfires but if your car had been running OK when this first occurred you may want to have the car properly diagnosed again so this cat doesn’t become defective…take the car back and let them fix it right if the ECM was not reprogrammed.

 

I use HP tuners and simply turn off all those damn emission warnings/controls and have never failed a BS state emission test. My vette is SC and I simply don’t want to deal with being rejected for a stupid emission check. After I’m done I go home and turn them back on again just in case I actually throw a real emission code.

 

When I got my Cats replaced and here in Florida we have no emission testing, and was told I need to drive it, had no lights returning has before, but we went on a road trip from Ft Lauderdale to Daytona average speed was 80 MPH driving at night hope the police were sleeping which they were lucky me.

All in that trip for week was over 500 miles, and when completed the smell of the new cats were gone, my gas milage in approved and had power once again.

I never had check engine light come on you might have another concern, since they had to play with 02 sensor they might have damaged it.

 

I took it to emissions testing first, later made an appt. to show the dealer the code that I got on the failed test. They did not need to diagnose which has already been diagnosed, unless they wanted to confirm.
The dealer did give me a 2 yr warranty on the part & service.
I know I need to set up another appt. and tell them what you mentioned about reprogramming the ECM.

And no, I won't be disconnecting the battery in the meantime .

 

Who diagnosed it initially ???…if it was the emissions testing facility did they just read the code and say…”you have a bad cat” ??…did they do any type of diagnostics ??..and tell the dealership to read their Service Info for once !!

 

Ok, I went to the emissions test WITH the MIL on, since it was running fine. And no they didn't tell me I had a bad cat, they said a code popped up, PO420 and that it failed the test. They told me to bring the emissions test paperwork to any dealer that I went to. I chose a local GMC place since they specialize in GM service.
Yes, I will remind them about the service info this coming Fri.

 

When I first failed the emissions test , I did research on here and read that some guys had codes pop up w/ the MIL on, was diagnosed by shops and told they needed to replace their cat(s) . Got it done & still had MIL on.
I'm basically in the same boat as them.
So when I first went to dealer last month, I told them what's up & made suggestions that the easiest (and cheapest) thing would be to check the O2 sensors first and replace if necessary. When they said the cat was bad and they recommended replacing it, I asked if they would replace O2 sensors w/ new ones if replacing the cat & they said no.

C5 Diag, thanks for shedding some light on this and for suggesting the dealer reprogramming the ECM, because I believe that should be the key to "fixing" this.

 

No problem !!…I’ll post a few videos below for you !!

 

Yesterday, I noticed the MIL went away. Drove the 'Vette to work today, still didn't see the light. So far, so good !
Hopefully everything is A-OK at this point & I can go to emission place & pass. 3rd time is the charm !

 

buy a 20 scan tool or goto autozone and use theirs