Another persistent P0420

[TomH91]

I could use some help fighting a persistent P0420 for bank 1 catalyst low efficiency, please. 2009 Yukon Denali 6.2, no DOD, 157,000 miles. OE replacement paper air filter is clean-looking. Truck is new to me and must pass emissions inspection before I can register it.

Long term fuel trims are strongly negative at -14.1 or more in closed loop.

Scan tool shows the rear O2s have nearly even voltage in closed loop and don't follow the fronts. Scan tool also shows expected coolant temp readings, sensor data looks ok there. Fuel pressure at idle measured by scan tool is around 43psi. Engine oil smells a little gasoline-y and I need to put a gauge on it and validate fuel pressure, but 43psi doesn't shout failed fuel pressure regulator.

I put on new plugs and wires. Indicated avg fuel economy went way up temporarily (16-20) but has since sunk like a stone, worse than ever at about 12.5 in town, down from 15.5.

Engine idle isn't the smoothest hot - better than before the plugs but not great. I need to get the scanner back on and check for misfires. The miss at idle seems intermittent so not pointing to a plug I maybe didn't get the wire seated correctly.

Next steps, in order:
1. look for misfires, check that wires are on tight. Swap coils if isolated to 1 or 2 and see if misfires follow. If they do, replace offending coil(s).
2. clean MAF sensor
3. borrow smoke machine from buddy, look for leaky intake manifold gaskets or other vacuum leak.
4. check fuel pressure at rail by external gauge. Look for evidence of internally failed fuel pressure regulator.
5. ask shop to check injectors with voltmeter (I'm not handy with one), look for bad injector(s).

I wouldn't try all that if the voltage on the rear O2s wasn't relatively steady in closed loop. If the voltage on the rears followed the fronts in closed loop I would have ordered a front Y with cats already.

Am I on the right track? Anything I'm overlooking?

 

[jz57]

Add followings:

Checking fuel injector leaking
Check any exhaust manifold broken bolt
Check exhaust leaking between up and down stream O2 sensors

Expensive way:

Install a known good down stream O2 sensor as diagnose purpose.

Install a new catalytic convertor will guarantee P0420/430 disappear for a while, but the root of the issue is not found.

 

[TomH91]

Thanks, good suggestions.

Couple of things to add:
Soapy water test on cold exhaust manifold shows leak at driver's side rear. Based on the sound, it seems to close up when warm. This may not be the only leak since I can't see the underside of the manifold from the top, with the hood up.
I replaced the driver side rear O2 sensor with new Delco.

Honest question: I've read in several places that an exhaust leak before the front O2 can cause problems with cat efficiency and negative fuel trims. What is the mechanism there? O2 doesn't see the input it expects due to an exhaust gas value being lower than expected due to leak, and ECM adds fuel?

 

[jz57]

Looks like you hit the nail head.

There could be a broken exhaust manifold bolt neat driver side firewall, that could be a leaking spot.

The extra O2 (Non metered O2, without going through MAP sensor) going to exhaust system, which triggers O2 sensors senses unburned O2, more like lean burn condition.

 

[TomH91]

Appreciate the thoughts. Since the exhaust manifold is positively pressurized while the engine is running, it doesn't seem like air would be pulled in through a leak, but maybe it could be drawn in between pulses? I'm reading some explanations like yours and it seems a leak will allow atmospheric O2 into the exhaust stream. Thanks for your post.

 

[swathdiver]

Time for a new cat. Check out Magnaflow for quality cats and well designed piping.

 

[Meccanoble]

Hi, I have a similar code and was looking at the scan tool. You mentioned the following:

"Scan tool shows the rear O2s have nearly even voltage in closed loop and don't follow the fronts"

While sitting at idle, mine shows voltage matching for 02B1S1 and O2B1S2. Whatever S1 volt changes to, S2 volt changes a second after. There is never a big difference between the 2. How can I test to see if an O2 sensor is bad?

I seen an O2B2S1 and O2B2S2 and this number seems to fluctuate a little more between each other. I took a screen shot showing a .5 difference at one point while the bank 2 sensor 1 and sensor 2 are rarely more than .1 difference.

I dont even know how many o2 sensors are on this truck, does that mean there are 4?

 

[Meccanoble]

Also I seen a state for Cat Bank 1 sensor 1 and Cat Bank 2 Sensor 1

Both show temp of 1022.0 F

 

[Meccanoble]

 

[Meccanoble]

Confirmed all 4 O2 sensors move between .1 and .9 volts. They are all moving up and down when driving. Cat Converter got as hot as 3150 F while driving. I did observe fluid coming out of exhaust but not at the end of the tail pipe like you normally see. Its leaking like right before the resonator, maybe under front passenger seat area of exhaust. I know exhaust leak was mentioned to cause issues. I wonder if this could be the culprit. Any suggestions?

 

[Just Fishing]

Probably going to be the catalyst in the end.
With the miles, it's probably at or near end of life

Many of us have also installed catch cans.
It's a little widely known these things can suck up oil through the pcv, and suck it right down the intake.

On my 5.3,
When i pulled the intake for the first time, a ton of oil started to pour out of it.

Seems like there is a catch inside of the intake, but after a while that would overflow i would assume...

Oil can poison a catalyst, part of the reason they started moving away from certain additives, and prompted roller lifters and stuff.

Now we have stuff like "hot rod oil", and "racing oil" that still have those additives for older cars.

I'm sure I'm over thinking it.

Also, did you happen to replace your o2 sensors lately?

If you do, replace all 4 at the same time.
Get oem, beware of repackage non oem in oem boxes.

 

[BG1988]

Most likely a restriction in the intake system replace the air filter most likely it's very clogged up or wrong one


I can ping the LTFT to far negative and misfire by adding an "artificial" restriction to the intake ....(i did this to ping test a FAR positive +15 sometimes spiked to +29 MAX open LTFT) (ended up being dirty fuel injectors ) I think the Royal purple is doing a good job i'm doing a 170 miles road test tomorrow better lower averages around 9-10 were like 40 miles so far

also check the MAP,baro(should match to key on) ,MAF and throttle body(could be dirty and sticking don't take apart the cog wheels just carefully open the throttle body flap and clean don't let it slam shut) make sure it works correct

it's easy to check them

 

[swathdiver]

Confirmed all 4 O2 sensors move between .1 and .9 volts. They are all moving up and down when driving. Cat Converter got as hot as 3150 F while driving. I did observe fluid coming out of exhaust but not at the end of the tail pipe like you normally see. Its leaking like right before the resonator, maybe under front passenger seat area of exhaust. I know exhaust leak was mentioned to cause issues. I wonder if this could be the culprit. Any suggestions?

Only the upstream sensors are supposed to move around a lot. The downstream sensors are supposed to stay fairly constant though they do move. When they move like the upstream, it means the catalyst is done for.

 

[TomH91]

As a wrap-up, if your money was on cats, you were right. New Walker stainless Y-pipe and cats and she finally passes emissions.

The coolant temp sensor data was valid.
The idle was smoother after I cleaned the MAF and the long term fuel trims dropped down to -4 to -6 range from -14.1.
I replaced the plugs and wires. #8 wasn’t terrible to do.
Air filter replaced.
No broken exhaust studs that I could find. Leak at rear of bank 1 closes up so fast and is so small the shop felt it wasn’t contributory.
I replaced the bank 1 rear O2 sensor with AC Delco.
Fuel pressure was normal at ide but wasn’t verified with a mechanical gauge when driving. I thought the pressure regulator might have failed internally but seems not to have been the case.
The fuel alcohol reading was over 20%, but I haven’t run any E85. The shop said the flex fuel engines sometimes have an issue where they get a bad alcohol reading and it persists and won’t reset properly. They have a customer with a 5.3 whose truck needs the reading reset every 3-4 months. They reset the alcohol reading on mine and the long term fuel trims dropped back to a normal range and stayed there.

Despite all the things I did and had the shop do, the cat efficiency code for bank 1 continued to crop up until I relented and replaced the cats. I was hoping to avoid the expense of new cats but with the miles the factory ones were pretty much aged out.

 

[Fless]

Good that you got it fixed, and thanks for posting the solution.

 

[Meccanoble]

Only the upstream sensors are supposed to move around a lot. The downstream sensors are supposed to stay fairly constant though they do move. When they move like the upstream, it means the catalyst is done for.

Thanks, the bank 2's definitely move a lot, maybe not as much as bank 1's but they definitely move. Anyone ever use Cataclean?

https://www.amazon.com/gp/product/B002BVXM92/ref=ox_sc_act_title_3?smid=A16TLDXIT2T97H&psc=1

 

[Geotrash]

Thanks, the bank 2's definitely move a lot, maybe not as much as bank 1's but they definitely move. Anyone ever use Cataclean?

https://www.amazon.com/gp/product/B002BVXM92/ref=ox_sc_act_title_3?smid=A16TLDXIT2T97H&psc=1

Yup. In my LR2. But it didn't work because the matrix had a huge hole melted through it from a bad coil causing a misfire and letting raw fuel get dumped into it. Happened before I got it, but the damage was done and cataclean can't fix that kind of damage.

 

[Meccanoble]

Yup. In my LR2. But it didn't work because the matrix had a huge hole melted through it from a bad coil causing a misfire and letting raw fuel get dumped into it. Happened before I got it, but the damage was done and cataclean can't fix that kind of damage.

I know for certain I see fluid leak from my exhaust nearer to cat, the same fluid that would probably leak out at end of exhaust. I ordered 2, I'll see if it helps. Thanks for sharing your experience.

 

[swathdiver]

As a wrap-up, if your money was on cats, you were right. New Walker stainless Y-pipe and cats and she finally passes emissions.

If you don't mind, post up the part number(s) used to help folks coming along later with these issues. Did you keep your old cats? I sold mine to the scrap man.

 

[Meccanoble]

Just want to share i added cataclean twice and after 200 miles, CEL has not come back yet and I can feel an improvement with driveability. Its like the exhaust got cleared or 02 sensors are functioning better. Less restrictions. Strongly suggest this before replacing any hardware.