03 H2 Hummer P0300 random misfire

Only code is a P0300 random misfire.

Did relative compression test and from what I can tell looks ok.

#6 has about 50 misfires and the history shows 2500.
#5 has about 20 misfires and history is 500

Changed plugs wires and moved the coils around and no change.

LTFT both banks is 25

Did crank relearn with zero difference.

O2 sensors have different readings. Dont understand what I'm looking at. Still learning.

Not sure if I'm looking at a bad cat on bank 1.

Fuel filter was clogged. Air filter was clogged. Replaced both and engine light is staying off but still showing random misses on all cylinders but mostly 6. LTFT dropped to 13 from 25.

Question is?

Do the Cats look bad and am I on the right track?

Any suggestions appreciated

On the second pic the -3 ; -3 are the STFT and the 9 ; 14 are the LTFT

This was after I changed the Fuel Filter and Air Filter

I went to tech school in 93 so Scanners were very new and I never used one.

I am trying to diagnose this Hummer without throwing parts at it. I've seen people with the same issue I have throw, cats, knock sensors, crank sensor, plugs, wires, coils, pulled the heads, and changed the filters and fuel pump and still have the P0300 code come back.

Luckily I've only done a full tune up and it's improved drastically. I have twice the amount of power. The small vibration at idle in the steering wheel is almost completely gone. Now it's only a very random vibration which shows on the scanner live data misfire as a miss. It's only missing about 10 or less misfires in a cycle.

I will probably pull all the injectors and flush them and the fuel rails. The amount of black soot that cam out of the filter was concerning. I don't know how many years it was on the Truck but I know it was rusty as heck.

Just want to learn more about O2 sensors and checking for bad catalytic converters.

So that's where I am at now.

Hey JupiterBandit! The easiest way to rule out a cat problem would be to do a wide open throttle run through first gear with the scan tool connected and recording. A highway on-ramp would be ideal. Save the file and either review the results once you've pulled over, or connect your Modis to your computer and use ShopStream Connect to review the data. It's free from Snap-On:

www.snapon.com/diagnostics/us/SSC/DownloadPage.htm

You can also upload and share the data capture itself so we can view it on our own.

You'll want to watch the MAF (Hz and airflow rate in g/s), MAP, Engine Load, Intake Air Temperature, both upstream O2's and fuel trims. Any breathing problem bad enough to cause a miss will be evident.

That said, your symptoms don't match a plugged cat, IMO. Given the mileage and the fuel system, I think you're more likely to find a restricted injector on #6. If you have a fuel pressure gauge, an injector balance test would be extremely easy to perform with your Modis.