Vehicle - 2008 Hummer H2 Base model 6.2L all stock no mods - 177K miles averages 14-17MPG town and highway just depends on my foot -

Whats up guys unfortunately ive been trying to figure this out for a couple months no luckily I don't drive a whole lot. However its now caused the P0420 code Bank 1 Below Threshold blah. blah. Now I know that's a (bad converter) but I also know cats don't go down on their own something upstream typically causes them to go bad.

Symptoms -
Rough Idle
No starting issues whatsoever warm or cold cranks right up thus far
P0420 Code
at idle RPMS are fluctuating slightly 550-650 or so
holding at 2000 RPMS with no pedal movement it drops off 50-150 RPMS on its own
so far no issues with take offs or anything like that no hesitations as far as I can tell


Now for what ive done to it so far....

Cleaned the TBS - and replaced the gasket as well
Cleaned the MAF - wasn't all that dirty TBH
all new plugs/wires - we never do just plugs around here
I have cranked and attempted to find any vacuum leak to no avail so far (still not sure if there is one don't have a smoke machine)
I own BlueDriver a OBD2 reader very minimal readings but all my O2 Sensors are getting readings and both Banks are very similar I did have a 02 Sensor code a couple months back could be related replaved the Bank 1 Sensor 2 down stream. Reset code and hasn't been an issue however I have had a rough idle for a while not and yes during that code as well just not as often or as rough of an idle as it is now. Its a little more consistent now. I have to get it fixed to I can then replace or clean the cats if needed.
Fuel System Test shows System A using all oxygen sensors as feedback for fuel control
System B has not yet satisfied conditions to go closed loop (bank 1 or both)

if you need more info let me know
No I haven't taken to a StealerShip or a mechanic I know several and have contacted several over the phone but they haven't driven or heard the issues themselves... that's my next step if I cant figure this out.
Any help you guys can share will be greatly appreciated I don't have major tools for compression testing or smoke machines so please bare that in mined.

Thanks
Mud

 

With the mileage it could very well be the cats are just getting clogged and need replacing.

 

you think a clogged cat could be effecting the engine at idle making it run funny? I had that thought but kinda figured it was probably more than likely the other way around. The engine running rough or missing or whatever its doing and then the cat got clogged as a direct reaction to the engines issue.

Thanks for the suggestion.
Mudd

 

Did you look the plugs over when you pulled them? Anything different about one from the other may tell you if there is an issue with a single cylinder?

The easy way to check the cats for plugging is to pull the pre cat O2 and screw a pressure gauge in to see what the pressure is, should be around 1.5 psi or lower at around 2000rpm..

Does your scanner have cat temps, my 08 shows cat temps which mine are running around 1200* normal driving (I have multiple scanners Torque, Dash Command, Tech2, etc....) Torque reads the cat temps?

I recently had rough idle issues and it turned out to be a bad purge solenoid for the charcoal canister. The tank pressure in pascals should be around 300 or lower at idle and when driving it will get as high as 2000. With the bad solenoid it was over 1600 at idle causing a rather large vacuum leak into the intake.

What do the fuel trims look like at idle and light cruise?

Sorry for all the questions just trying to get an idea of what you've looked at.

Neal

 

Look at the throttle position when idling, the h2 is DBW so the throttle will be open a good bit to compensate for idle, mine is around 20% if much lower than that you may have a possible vacuum leak.

 

I'm not sure but the ECU may be using the cat temps to go to closed loop so a bad cat may cause this.

Mine goes to closed loop pretty fast after crank up, the cats heat up pretty fast.

With the heated O2 sensors they heat up fast also, maybe swap the O2's and see if the closed loop issue moves to the other side. This would let you know its not an O2 heater issue.

Just throwing ideas out there,

Neal

 

Dang guys thanks for all the ideas and input. I recently had a buddy look at it that also works on his own vehicles and he noticed since it was late the headlights were on when the RPMs would idle down slightly seeming like a miss to me the lights would also dim and then go back to norm after the idle returned to norm every time I had never checked the lights I was always messing with it during the day time. I have original head lights no aftermarket anything at all.

im going to use the bypass on the 02 Sensor for now till Jan when ill be redoing the whole exhaust my buddy also is pretty sure theres a slight exhaust leak so plan to get that repaired then as well.

I will also try swapping the 02 Sensors as you mentioned and reset it and see what happens.
Thanks again guys for all the ideas.

My question is if the lights are dimming as well during this idle issue could it be the alternator I did just have a Parasitic draw for a while that ran my battery dead multiple times and so I replaced the battery and got that mess figured out (onstar module) go figure just pulled the Fuse and bam no more drain so. Could it be the alternator effecting idle I haven't had any issues starting the truck even after a week straight with no starts it just fired right up? so I don't know

 

I did have a code for the purge valve I replaced everything except the charcoal cannister I blew it out till the dust stopped completely not sure if it will work but I got it all working again no leaks and then boom no more code went out on its own next driving cycle.

both cats are 1238-1258 on my bluedriver
all four 02 sensors show readings and both normal conditions so idk about that.....

 

cat temps look good,

I ended up pulling my charcoal canister and cleaning it also, what a job, they didn't place it for easy removal

are the fuel trims close at idle?

If you could find someone with a Bi-Directional scanner they could do a injector balance test for you.

The open / closed loop thing is usually an 02 sensor heater thing or a bad ECU engine temp sender. Possible bad wire to the 02 sensor heater circuit or a ground issue.

Neal