turbojimmy

2007 Avalanche, LC9 engine. I've had a P0420 code for a long time now. Check Engine/Malfunction Indicator Lamp (MIL) light has been lit for months. It's due for inspection in January so I need to fix it. MIL lit up is an automatic fail. My code scanner tells me the code, as well as the status of the MIL - which is "ON." Today the MIL went out. Did it magically repair itself?

I plugged in the scanner and it still has an active P0420, but the MIL is now "OFF." I don't know what to make of this. The state inspection station plugs into the OBD-II port to check for codes but since the MIL isn't lit I don't know whether it will pass or not.

If I need to do the work, I'd rather do it now than when it's really cold in January.

Has anyone seen this before (P0420 but no MIL)?

SBchevyfan

I take it that they cleared the code when you read them. You may have what's referred to as a pre-code. Try having the codes cleared again and then disconnect the battery for about 10 minutes. After reconnecting the battery, have the computer scanned again for codes and see if the code comes back. In Wisconsin, they look to see how long ago the codes were last cleared. You need to put on about 150 miles in Wisconsin to get the emission test run.

turbojimmy

I usually clear the codes after I pull them. This is the first time the light went out on its own, though. The light always came back on after some period of time. I assume the light is coming on as soon as the conditions of catalyst drive cycle are met, which will vary depending on my schedule. I read NJ's rules for OBD-II testing. They're not really clear, but it sounds like they can fail you even if the MIL isn't on. If you clear the codes and come back, you're allowed to have one monitor not ready. But if you fail for a catalyst malfunction specifically then the catalyst monitor has to be ready. Currently my EVAP monitor is showing not ready, which is even more strange. I would expect no MIL with a P0420 if the catalyst monitor wasn't ready. I'm just going to clear the code again and see if it comes back. It's just strange to see no MIL with a stored code (it's not a history code either- it's an active one). And no the light isn't burned out . The light comes on with a KOEO test and the scanner indicates the light is off.

tech2

the monitor can run and pass; turning off the mil...the code will still be stored in the module....it may need to run and pass a number of times before the module clears it from memory...some parameters require 40 trips before it is cleared from memory; without the use of a scan tool to clear it.

a p0420 is usually due to a tired old cat. Slow switching 02 sensors can cause it also. Installing new rear 02's and having slow switching front 02 sensors can cause the fault also. If the 02 are switching properly, replace the cat. Don't cheap out and put in a cheap cat either. Low cerium content from a cheap cat will cause a immediate reset of the p0420

for the state inspection, no dtc can be present and all i/m monitors must have run and passed.

turbojimmy

Thanks. Hopefully it won't come back. I suspect a bad cat. I had the PCV issue where it would blast oil into the intake when switching from V4 to V8 mode. I don't know how long it was doing that - a friend of mine noticed when he was behind me on the highway - huge blast of blue smoke. I installed the revised valve cover and fixed it, but I think it might have damaged the cats. I was looking at a Walker-brand kit with 2 new cats. The GM part is twice as much and frankly I'm just trying to nurse this 170k mile truck along. I don't want to spend a ton of money on it.

NJ lets you get away with one not-ready monitor so long as it's not a catalyst monitor if you've failed for the catalyst previously. I usually go through with a not-ready EVAP monitor because they take for ever to reset. Whatever the conditions for that are, I take a long time to meet them. I very rarely drive on the highway or have the truck above 50 MPH.

tech2

the evap monitor is allowed to not complete is some areas...the evap on some makes are temp dependent...the monitor wont run if the temp is to cold. That is why you dont get small evap leak codes in the winter

turbojimmy

So the light came back on as soon as the catalyst monitor was ready. I'm going to bite the bullet and put new cats on it.

And now it's developed a pretty solid misfire - but only at idle. Threw a generic P0300 (misfire) code. But that one does not turn on the MIL so it should be okay if it doesn't clear up by inspection time.

tech2

dont get weld in cats ...get a complete y pipe

turbojimmy

Yes, I definitely plan to get the whole y-pipe. Maybe it will magically clear up my misfire, too....

tech2

if the cats are clogged or have loose, broken pieces of substrate, it will cause a misfire.