starfishy

A couple of days ago my '00 Camaro SS with 84000 miles on it threw a P0420 code (Cat effectiveness Bank 1). The Torque app shows that the O2 sensor readings for the downstream O2 sensor of bank 1 are normal for a little while, then suddenly crash and after a while recover, sometimes fully, sometimes they crash again. The right bank downstream sensors remains relatively flat. I cleared the trouble code, but it came back within minutes.

The other thing I saw were high misfire rates in torque. Not sure whether the two observations are related or not.

I replaced the O2 sensor (cheap, was hoping this might fix it) and reset the code, which stayed away almost all of today. Got the car smogged and 'passed' but as I hadn't driven the required 200 miles after code reset (I didn't know about that) have to go back. And the Camaro threw a P0420 again in the late afternoon. I replaced the spark plugs on the driver side because I saw high misfire readings in torque and found the gaps in the old ones were huge, but the behavior of the o2 sensor did not change.

Now the question is, is this the cat (I thought that if the cat is damaged I should feel sluggishness at WOT and also if the cat was damaged wouldn't the readings be universally low instead of going up and down? But what could it be? Wiring? Spark plug wires? Something entirely different?

A Walker Cat is $320, so that wouldn't be breaking the bank, but I would like to avoid spending them if it turns out not to be the cat.

The car seems to drive fine, though I think my mileage is pretty bad at the moment. I saw a slight improvement in engine smoothness after I replaced the O2 sensor, but that could be my imagination.

The sensor I used is a Denso 234-4025 which supposedly fits my car. It physically fits and plugs in properly. Spark plugs are NGK TR55 Iridium.

I'd appreciate any advice and opinions.

O2 sensor readings:


Left: Bank 1 upstream vs downstream, right: Bank 1 vs Bank 2 downstream.

Germansheperd

That graph tells me you have 1 good working O2 and 1 that is shot.
At 84k the all the O2’s, plugs, wires and possibly the cat is shot. Generally the 0420 code is a bad cat. The Magnaflow cats are decent as well.

starfishy

I swapped the left and right O2 sensor. The graph still looks roughly the same (right-good, left = bad. I replaced the 4 spark plugs on the left, the 4 on the right are going to be replaced next weekend. I know #8 is the difficult one. Spark plug wires are on order. I think the spark plugs took care of the misfire (didnt feel that, but saw the counters). After the reset the misfire seems to be gone. Let's see whether that stays. I have not had a new P0420 yet, gonna drive the car to work tomorrow to see whether the code will return. If the wires dont make a difference Im going to order a Cat. Magnaflow is a lot more money than Walker (320 vs almost 500, is the Magnaflow cat that much better?

Germansheperd

I really cant say one is better than the other. The Magnaflow hi-flow cat I bought for my LT1 is a beautiful piece and I’m very happy with it.

DrewHMS97SS

What mods are done to the car?

starfishy

Engine and drivetrain are stock except for an LS7 clutch and the airbox lid.

JakeRobb

FYI, GM issued an extended warranty (10 years, 120k miles) on the cats in all LS1 F-cars due to premature failure. You’re obviously well outside that range. My first one failed around 90k, and the second just a couple months later, still in the low 90s. This was in maybe 2009 (on my 2002), so GM replaced them both for free.

You’re out of luck with respect to the free replacement, but this lends significant credibility to the idea that one of your cats has gone bad. You should save yourself the trouble and replace the other one too, since it’s sure to fail soon.

EDIT: Any chance you’re located near Michigan? I’m installing Kooks headers and their catted Y-pipe, which means I have a stock Y-pipe with relatively fresh cats (~25k miles on them) available. One of the flanges broke off where it meets the factory manifold (which prompted the upgrade) and will need to be welded back on; a simple job for someone with the equipment and skills. I’ll give it for free to someone who can come pick it up or meet me nearby.

Injuneer

Which bank are the misfires on? Was the SES light flashing? Did you feel the engine misfiring?

A misfire dumps raw fuel and air into the exhaust. The O2 sensor sees the air as high O2, the PCM assumes that bank is running lean, and adds extra fuel with the fuel trims. That fuel/air burns on the cat, overheats it and damages the cat. The heat can crack the substrate and blow the innards out the exhaust pipe. Get it hot enough, and the substrate can melt and limit flow. It is the last extreme that you would feel as a loss of performance, so apparently you didn’t reach that stage.

Suggestion - use only OEM/AC Delco O2 sensors. GM cautions in the factory service manual that aftermarket O2 sensors can cause problems with the cat efficiency diagnostics.

starfishy

@JakeRobb @Injuneer Thanks for your insights!

Here is the newest, from today: All emissions tests readiness values show green, except for Secondary air system, which shows incomplete. This includes misfire (also green). Also the graphs for misfire in the Torque Pro app show a solid zero for misfires for all 8 cylinders. In the 'vehicle test results' section I still get the TID: $0c CID: $30 misfire count (calculated) Not OK. II replaced all spark plugs on the driver side and will replace all spark plugs and the spark plug wires on the passenger side on Saturday.
The car still drives as normal, no codes, no SES light.
I also did not get any of the misfire trouble codes (P03xx).
Ill also clean the EGR just to be on the safe side.

If the cat turns out to be bad its going to be ~$300+tax, I can live with that, I would just be mad at myself for spending $300 to replace a cat and still be saddled with the same problem, so I am trying to address everything I can before thinking of the cat. Its been several days since it threw the last code, so I can only wait and see.

starfishy

Looks better

The P0420 has not returned, but today I got a pre-fail on P1415 (AIR Bank 1). As I didn't have replacement AIR check valves I unmounted them and cleaned them out. Before both were plugged to the point where no air would go through, after cleaning them out there was some resistance but they do flow air. I drove ~100 miles and the only two items that did not clear in the Torque app's emissions readiness screen were cat heating (I infer that's because I cleared the P1415 on a warm engine and haven done a couple of cold starts yet - and evap (Not enough stop and go?) Notably catalyst and secondary air system are complete. I read I can pass smog with evap incomplete, but I'll drive to work the next couple of days anyway, lots of lights.

Btw, I got the P1415 a few times before, normally i reset it, but I think I should replace the AIR check valves after all.

starfishy

Off to the smog station tomorrow!

Cleaning out the AIR check valves did make a difference in driveability, too, especially at low rpm. 50mph in 6th sounds smooth now where it sounded a bit uneven before.

Edit: I drove 150 miles since the reset for the P1415, so I hope this is long term fixed now.

starfishy

The Camaro passed smog. Cat was apparently not the problem. Neither were the O2 sensors. I had all the smog monitors green, then it threw another code before I went to smog it again. P1415.

Secondary AIR injection driver's side. So I unscrewed the AIR check valve, which was full of soot and entirely plugged. Not having a new one on hand I cleaned it with carb cleaner until it flowed air again.

After that the smog readiness monitors turned green and stayed green - until I actually had it at the smog station. I checked shortly before and everything was fine, then it was hooked up to the machine and a few minutes later the smog guy told me all sensors were incomplete again. No codes. I drove it for another 40 miles until everything was green again. This time it passed with flying colors.

Since then no problems. I am still going to replace the spark plug wires and I have two new AIR check valves, which are going in tomorrow, as will the right side spark plugs.

Thanks again guys!