Another persistent P0420

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?

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.

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?

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.

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.

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

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?

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

Both show temp of 1022.0 F

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?