P0420 Catalyst System Efficiency Below Threshold Bank 1 (Help)

"Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94) 08/22/2018 at 09:10 • Filed to: None

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(Video placeholder for thumbnail because picture uploads are f’d for me)

So, I had my second ever idiot light this morning. First one was about 2 weeks ago. Both were P0420 codes, so presumably there really is something wrong since it came back. The facts:

- 2010 LH9 5.3L V8

- 125,000 KM

- I didn’t give it any warmup time this morning, just started up and left (but obviously didn’t beat on it cold). Don’t remember what I did the first time the light came on. I don’t see how this is an issue as I’ve done it at least 40-50% of my cold starts, but there it is.

- Both times the light came on in almost exactly the same place, about 3-4 minutes out of the driveway. And once cleared didn’t come back.

- The lights came in different tanks of gas (back to back)

- Could !!!error: Indecipherable SUB-paragraph formatting!!! be related? It died down through the summer, I also ran some of the additive recommended in that post through it. Now that the mornings are slightly colder it’s back. Dealer could not replicate in April.

- O2 sensor voltages - Upstream fluctuating as normal, downstream steady at ~0.745v. Both sides.

I know the primary cause of that code is a bad cat... 125,000km seems pretty early. What else should I check on?

EDIT: I also have !!!error: Indecipherable SUB-paragraph formatting!!! going on that I’m not sure what to do about. Could be a factor.

sony1492 > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/22/2018 at 09:20

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The only way I know to see cat efficiency is to get it on a lift, hold it at 2k for 2 minutes and then measure the difference in exhaust temp right before and after the cat. The temperature should be higher after it.

Smallbear wants a modern Syclone, local Maple Leafs spammer > sony1492 08/22/2018 at 09:23

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OK, one vote for mechanic. The best I’ve got is a thermal imager that would go out of range.

crowmolly > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/22/2018 at 09:33

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3-4 minutes might be when the car enters closed loop.

That O2 is probably cheap if you want to try replacing it first. Also, check with a local dealer. I seem to remember cats being an 80,000/8 year warranty part. If Stigness is out there maybe he can chime in.

Smallbear wants a modern Syclone, local Maple Leafs spammer > crowmolly 08/22/2018 at 09:48

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I’ll take a look at that. I’ve got an UltraGauge so I can see exactly when that happens. What would that indicate?

The problem I have with replacing the O2 is that I’m pretty sure I’d wreck something else on the way out. When I had the t ruck tuned we had to try all 4 sensors before we got one that would actually come out. I’ve always planned on eventually doing a full exhaust so I wasn’t too concerned, but I wasn’t planning on doing that quite yet... then again, if it IS the cat, I might as well get it over with, and all 4 sensors at the same time. O2 voltages seem to indicate they are working though

crowmolly > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/22/2018 at 09:54

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When the engine is first started it will be in open loop- sensors are cold and the ECM ignores O2 sensor stuff as it’s not reliable yet. Once they get up to temp the ECM enters closed loop and will use O2 input to adjust the tables. You are probably hitting temp at around the same time.

Just spitballing but I’d bet that the cat itself is fine and the O2 may be fouled.

Spamfeller Loves Nazi Clicks > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/22/2018 at 09:57

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You got an OBD-II reader that’ll show your STFT and LTFT?

Honestly, pre-cat behaving normal and post-cat reading at a flat 0.745v for both, I don’t think it’s mechanical. I think it’s electrical. Either sensor or PCM.

Smallbear wants a modern Syclone, local Maple Leafs spammer > crowmolly 08/22/2018 at 10:00

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Wouldn’t that cause the voltage readings to be different for each bank though? Or, alternately, cause each bank to have the same issue if they’re both fouled?

Worth a look in any case . I’m not looking forward to trying to get those things off though...

Smallbear wants a modern Syclone, local Maple Leafs spammer > Spamfeller Loves Nazi Clicks 08/22/2018 at 10:05

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UltraGauge. Yeah, it’ll show that. I checked it once so I know it’ll give me that reading... good call on checking it again. I’ll just have to look up what’s normal again, not a hard thing to check.

Post-cat being flat is also normal. The voltage itself is a little high (rich) though... but the truck is tuned so I figured that was probably what it was picking up. It is something I’m keeping an eye on though. Come to think of it, I checked those readings cold... I’ll have to check them hot.

“or PCM”

Bloody well hope not...

Spamfeller Loves Nazi Clicks > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/22/2018 at 10:22

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Well, pre-coffee, I’m bad at articulating. Like, “or PCM” meaning there’s a ton of possibilities, not all of which are “the PCM’s bricked.” Most of them aren’t that.

However, here’s the thing - post-cat being flat with a P0420? That’s not right. You should be seeing fluctuation to set the P0420. Also bear in mind that you have 4-wire sensors. If you have a heater element failure, that can set P0420. (Also remember that P0420 is bank 1. So you have one side bad, one side good.)

The other telltale is that you don’t have a P0421, warm up catalyst below threshold, or any other catalyst codes set. More importantly, you don’t have a P0422, main cat below threshold bank 1. What I’m expecting you to find here is that STFT and LTFT are both stable.

If STFT/LTFT are stable, and no other P042x codes are present, the assumption should be that the d/s lambda is faulty. Confirmation is to swap the u/s and d/s sensors where possible. If the u/s and d/s read the same after swap, you need to check the 12V PWM supply for the d/s sensor. It may also be a faulty u/s sensor. If you can’t swap u/s and d/s, cross-swap u/s between banks.

The other big question after that is history. If the truck had a long period of unresolved misfire, that would break the cat early. If it was just worn out, you’d have a P0420 + P0430 (bank 1 + bank 2) and significant variance. If you’ve got both banks reading very similar and stable STFT/LTFT with only one bank 1 code and no bank 2 codes, proceed on the assumption of bad sensor.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Spamfeller Loves Nazi Clicks 08/22/2018 at 10:50

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I mean, I highly doubt it would be. But I hadn’t even thought about it as a possibility, so... yeah. Bloody well hope not :)

The downstream O2 sensor produces a relatively steady voltage reading of approximately 0.45V if the catalytic converter is functioning properly. If the voltage of the downstream O2 sensor is constantly jumping between 0.1V and 0.9V, the catalyst is worn and the catalytic converter needs to be replaced.

^ Found from several sources, though with different voltages considered “normal”. Do you mean the fact that the code was set should mean there was an irregularity? As far as it being on Bank 1, that’s another thing that’s throwing me here... the voltages on Bank 2 are exactly the same. Anyway... how would I check heater failure? Pretty much just parts cannon at that point?

I’ll check the trim numbers... you can expect to hear back from me on that :)

Oh shit. One of the problems I have here is that the sensors are pretty much stuck. I had the truck tuned a couple years ago, we had to try all 4 before we finally got one out. On bank two, so that doesn’t really help. I’ll have to have another go at them if it comes to that I guess.

It has a history—as all these trucks seem to—of constant knock retard straight out of the box. Between 89 gas and the tune much of it was eliminated but I’m pretty sure it’s still occurring sometimes. Suspected culprit by the guy who did the tune was the exhaust—GM apparently strangled this thing deliberately... exhaust went into the muffler @3" and came out @2 1/2". But I’ve never confirmed it. As far as misfires go, not as far as I’m aware. I know I’ve had *A* misfire sometimes, but very rare isolated incidents. I can remember 3 the whole time I’ve had it, two were presumably caused by a wet air filter (recently washed AEM, apparently not fully dried. Pulled it and gave it another day to dry, didn’t have the problem again).

Spamfeller Loves Nazi Clicks > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/22/2018 at 11:11

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There actually isn’t “one true lambda” for O2 sensors, just like there isn’t one true sensor element. The SSJ is equipped with cats, breezes through a tailpipe test, and the expected d/s value is 0.462-0.467V but may be as high as 0.525V cold. Lambdas can get really weird, really quick.

The fact that it’s Bank 1 only is why I mentioned PCM. If voltages are clean on both, there’s no other codes, and the only code is a P0420? That’s not right, and it’s not mechanically possible. Could be the PCM is coding incorrectly and you actually have a P0030 or related. Heater failure check is, as I mentioned, just a matter of swapping the existing O2 sensors around. Throwing a new part at it here is the wrong answer. First swap what you have around to see if the problem follows. If you throw parts at it, it just makes it harder to figure out.

Also note that even after you swap parts around, the code should stay set. P0420's a hard code, so it stays set until you’ve completed 3 closed loop drive cycles without fault. GM doesn’t have a fixed drive cycle table either. But unless you’ve got smog coming up, you’re probably safe to force-clear it.

If they’re truly stuck, honestly, they likely were damaged back then. Over-tightening O2 sensors will cause problems. So they’re gonna have to come out somehow. Also, make sure you use proper anti-seize when installing them and get absolutely NONE on the sensor assembly itself.

Past that, I’m having to lean on my base of expertise rather than any engine-specific expertise here. I’ve not touched a GM V8 in a long time, and honestly I don’t think I’ve ever touched the LH9.

As far as tunes though, any time somebody says they’re just taking a WAG at it and blaming the exhaust randomly? Yeah. I wouldn’t trust the tune. Either you know or you don’t. And if you don’t, then even with GM’s self-adjustment, you’re gonna set it up wrong. Can easily get it into what I call ‘fuel dump’ mode where you’ve confused it enough to just run the injectors wide open.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Spamfeller Loves Nazi Clicks 08/22/2018 at 11:36

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OK,

OK,

Here I’ll have to wait for it to come up again. Google consensus seemed to be clear it, if it doesn’t come back on inside a few hundred km it was a one-off glitch. I cleared it today too. Because not the brightest bulb.

Yeah, I remember at least one would back out halfway and stop. So someone’s probably given them an extra tweak or two at some point. Unless it’s just the heat cycle that jammed them up? Either way they were in pretty good and I didn’t feel like breaking one off an hour from home.

OK,

To be fair, he acknowledged it as such. Strictly an after the fact “In my experience such and such this does not constitute medical advice” sort of thing. FWIW the Silverado with a 4.3 has 3" exhaust all the way back right from the factory... a Colorado 5.3 having a 3" get cut down to 2.5" seems really odd. GM has pulled this deliberate strangulation before, too. Ceasefire 305 (tiny intake head ports, normal intake) springs to mind, though it was a long time ago. The tune has run for 2 years now really nicely, and no real change in fuel economy so I doubt it’s bad. I’ll k eep an open mind about it though.

AMGtech - now with more recalls! > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/22/2018 at 15:40

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I’ve seen tunes kill cats prematurely. Running rich, the cat  works harder than it should, and if it can't keep up it could eventually start burning unburnt hydrocarbons and melt the monolith. That's an extreme scenario, but it does happen.

Smallbear wants a modern Syclone, local Maple Leafs spammer > AMGtech - now with more recalls! 08/22/2018 at 15:43

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Could be it. At the same time, because of the knock issue he couldn’t really do a ton to the tuning. More area under the curve, but not a lot of actual peak gain... so I don’t think it would be extreme enough for that. Which isn’t to rule it out.

AMGtech - now with more recalls! > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/22/2018 at 15:47

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Not necessarily, particularly if the PCM doesn't monitor O2 heaters and just runs them based on time and goes into closed loop based on other sensor readings. If you have bad O2 heaters their readings won't be reliable, but if the PCM doesn't know the heaters are bad it'll still use those values. And if your scan tool isn't fast enough it might not pick up bad readings. Like how a voltmeter is an average reading over time, it can't pick up rapid fluctuations. A quality oscilloscope is needed for that. Actual values in a scan tool, especially a cheap one, on a domestic car, are not the end all be all, they're a guideline that will only get you so far.

AMGtech - now with more recalls! > sony1492 08/22/2018 at 15:49

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More of an art than a science for the inexperienced, especially if the cat is borderline. But definitely a test worth doing.

Smallbear wants a modern Syclone, local Maple Leafs spammer > AMGtech - now with more recalls! 08/22/2018 at 16:00

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OK, I wonder if one of the heaters is on the way out then. Whenever I look, the numbers are good... wonder if maybe the sensor isn’t totally up to temperature when it goes closed loop and is fine shortly afterwards.

AMGtech - now with more recalls! > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/22/2018 at 16:03

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You could always do a continuity check of all the sensor heaters. If one is open or vastly different from the others...

Smallbear wants a modern Syclone, local Maple Leafs spammer > AMGtech - now with more recalls! 08/22/2018 at 16:13

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How would I go about that?

AMGtech - now with more recalls! > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/22/2018 at 17:04

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You need an ohmmeter. Disconnect the O2 connectors. One lead goes to one heater pin, and the other lead to the other pin. Voila. Really simple. Might need a diagram to determine which pins  are for the heaters. I'm sure YouTube has some tutorials if you have trouble.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Spamfeller Loves Nazi Clicks 08/23/2018 at 08:34

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After my commute home, O2v were steady on the rear sensors. Bank 1 was slightly lower than Bank 2, which I hadn’t noticed before. That continued this morning, with Bank 1 reading ~0.2-0.4v lower than Bank 2. Only exception was immediately after startup where they were even at 0.45v.

The following are hot readings:

Fuel Trim, Bank 1, Idle-> Short: 0 to -2.3 Long: 5.47

Fuel Trim, Bank 2, Idle-> Short: 0 to -1.6 Long: 3.91

Fuel Trim, Bank 1, 2500RPM-> Short: Roughly -5 to -7 Long: More or less steady at 1.5 for a bit, it dropp ed to 0.78 literally a split second before I let off.

Fuel Trim, Bank 2, 2500RPM-> Short: Roughly -5 to -7 Long: More or less steady at 0 for a bit, it dropped to -1.6 literally a split second before I let off.

Maybe I should hold 2500 a little longer and try again?

Smallbear wants a modern Syclone, local Maple Leafs spammer > crowmolly 08/23/2018 at 08:35

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I enter closed loop in about half that time, as it turns out. No dice.

Spamfeller Loves Nazi Clicks > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/23/2018 at 10:24

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Nah, this is what I was kinda afraid of, honestly.

These numbers are perfectly fine. Completely and utterly fine. There is no red flag here, no warning signs, no nothing. Actually says that bank 1 is a little on the lean side, but not significantly lean.

Normally someone might say “woah, 29% between, something’s up.” But there’s a bit of an art to interpreting these numbers. You have 29% between banks, but when you look at full scale (-50 to +50 or -127 to +255) the real difference is much narrower. Bank 1 @ 10%, Bank 2 @ 7%, real difference <=3%. That’s “slightly dirty intake” level at worst.

Then I gotta do the whole blah blah total trim, and the result there is still within the 10% total trim range. Even if we assume worst case (why can’t everyone just gimme raws? Fucking OBD...) you’re in 10% trim. Given year and miles, it gets the 15% allowance, so it’s actually in good trim. LTFT is supposed to swing more than STFT (LTFT defines STFT range ) so it’s in good shape.

The problem is, with that tune, we don’t know what the trim versus absolute versus baseline actually is. GM has a really advanced auto-tuning setup in the PCM though, so generally, unless you are hellbent on just fucking up the map, it’ll gracefully handle most anything.

All that plus the absence of any other codes at all, I’d say just leave it be, see if the problem recurs. This is really looking like a classic “$150 drivability” issue - MIL with no mechanical. Could’ve just been a bad batch of gas.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Spamfeller Loves Nazi Clicks 08/23/2018 at 12:04

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Thanks, that helps. I appreciate your long-distance troubleshooting :)

One more thing...

Since I posted this I’ve been paying closer attention, noticed a little stutter on say 3/4 throttle/3000-4000RPM getting on the highway. Not every time. And I’m pretty sure it’s something I’ve glossed over because it was intermittent rather than a new symptom.

A cousin of mine had a really bad misfire in their F150, any time they floored it the thing just fell on it’s face. Eventually they changed the air filter and problem solved.

My brother had a car where the O2 and MAF were fighting each other because the MAF was fouled. It had an intermittent CEL and driveability issues. Problem was an over-oiled K&N.

Now I have a AEM filter I washed about 4 months ago. It HAS seen some dirt, even though I wasn’t usually behind someone it still could have some buildup.

And as per the last link in my post, I have a similar issue to an unresolved post in a forum where there was oil or something being sucked into the intake. The guy didn’t say there were driveability issues but it could potentially be gumming up the MAF a bit.

I dunno, it’s just a wild-ass shot in the dark. You’d know better than me whether that would make any sense at all to take a look at. I’m going to let it ride for a bit and see if the light comes on again.

As far as bad gas goes, like I said this occurred on two different tanks. That said, I run my tank all the way down before filling, so it’s possible that isn’t doing it any good... but it hasn’t caused any identifiable issues thus far.

Spamfeller Loves Nazi Clicks > Smallbear wants a modern Syclone, local Maple Leafs spammer 08/23/2018 at 14:59

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My pleasure! Figure I may as well put the knowledge Dumber-Chrysler paid for to good use, right? :P

MAF can introduce all kinds of fuckery, but actually, that wild-ass shot might not be so WAG. Bank 1 is running just a smidge lean compared to bank 2. Some imbalance is totally normal. (Has to do with block temp variances, coolant flow, combustion temperatures, air paths, etc. The engineering involved is just mind-bending.) But generally, MAF problems, you’ll see both banks with a code or issue.

But the mention of oil being sucked into the intake got me thinking. I would, just in case, check your PCV and intake. If the PCV’s got a problem, it can do some weird things. Chances are it’s fine, but, since it’s a fairly easy to access part and cheap if it’s iffy, worth a quick look . PCV tends to go single bank.

But I really think this one’s gonna end up in the “looked everything over, it all checked out, couldn’t reproduce” pile.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Spamfeller Loves Nazi Clicks 08/23/2018 at 15:25

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:)

I had the same thought about both banks being affected, if I’m honest. But at the same time, the F150 I mentioned had no codes at all, so I lumped it in the “maybe” pile.

OK. Definitely worth a look.

Most likely, since it seems intermittent... Also, on an unrelated note you reminded me of this.