2008 SIlverado 4.8 P0420 Code but has good Catalytic Converters

[Brandon Longerbone]

Hello, I know this code is fairly common so I apologize for starting another thread on it, but none of the info I've found has helped. I have a 2008 1500 Silverado with a 4.8. Had it for 4 years and it's run great, until today when I got the code. I know that 95% of the time its the catalytic converter, but I've tested mine and they seem fine. Using a scanner, the O2 sensors read as they should based on what I've found. Both upstream sensors fluctuate as they should between 0 and 1 volts, and the two downstream sensors remain consistent as they should, fluctuating between .73- .77 (Bank 1) and .77-.8 volts (Bank 2). I tested that both at idle and at 2200 rpm. A temperature check of the cats shows about 430 degrees on the intake and 550 or so on the exhaust. Tested this at high RPM also.

 

I am missing a couple of exhaust manifold bolt heads, but I have no sign of a leak, runs quiet too. I was thinking about spraying soapy water on the exhaust when it cools down and then running it to see if there are any leaks. Other than that, I'm at a loss. I mean the readings the O2 sensors are giving seem to imply they are fine too right?

 

This is my daily work truck, and I want to get this figured out as soon as I can. I smelled gas a couple times today, and I know that if my cats aren't bad yet, they will be soon if it runs rich. Actually had this same problem on a 2000 Silverado before this truck. Replaced all O2 sensors, cats, and manifold gaskets chasing the problem and never could get it to stop running rich and showing the code, so I'm very motivated to solve this before something worse happens. Don't even mind spending the money or doing the work, just can't figure out whats wrong...

 

Anyone have any idea what might cause the code besides the cats and O2 sensors? Or do my readings actually mean something is bad?

Edited July 3, 2020 by Brandon Longerbone

[evad]

check plugs and wires 

[CamGTP]

What does the fuel trim data say?

 

Were you looking at those numbers too when you check the working condition of the sensors?

[evad]

i didn't know you could get a 4.8 in a gmt900

[evad]

maybe you mentasay

4.3?

[CamGTP]

The 4.8 was in trucks all the way to 2013.

 

Even longer in the chevy and GMC vans.

Edited July 3, 2020 by CamGTP

[evad]

i didnt even know that ...lol 

[evad]

anyway i hope he gets it fixed 

[Brandon Longerbone]

2 hours ago, evad said:

check plugs and wires 

Both were replaced a few months ago. Engine runs fine, no misfires if that's what you were thinking.

[Brandon Longerbone]

3 hours ago, CamGTP said:

What does the fuel trim data say?

 

Were you looking at those numbers too when you check the working condition of the sensors?

I have some of it recorded here but honestly, I don't know enough about fuel trim to know what it really means. Long term stayed at 0 to 0.8 during idle. More or less the same for short term. Though a couple times the short term went as low as -14%. I know you ideally don't want to be outside a -10% to 10% range but, I don't know how far off that really is.

Edited July 3, 2020 by Brandon Longerbone

[CamGTP]

You'd only be worried if the long term data was very high is either direction. Very positive trims would be a lean condition and very negative would a rich condition.

 

Short term trims are going to dance a ton because they react faster than you can blink to changes in the exhaust. Seeing random high positive/negative value is kinda normal because there are lots of variables going on. Like getting on the throttle real quick or being steady state and lifting off the throttle, there will be a change in commanded fueingl and the short term trims are going to show that instant change.

[Brandon Longerbone]

1 hour ago, CamGTP said:

You'd only be worried if the long term data was very high is either direction. Very positive trims would be a lean condition and very negative would a rich condition.

 

Short term trims are going to dance a ton because they react faster than you can blink to changes in the exhaust. Seeing random high positive/negative value is kinda normal because there are lots of variables going on. Like getting on the throttle real quick or being steady state and lifting off the throttle, there will be a change in commanded fueingl and the short term trims are going to show that instant change.

Gotcha, so then it seems things are normal there then. I did just realize my fuel pressure is consistently 43-44 psi though. I don't know if the scanner reading is as accurate as an actual pressure gauge would be, but the fuel pressure should be more in the 50-60 psi range no? Then again I'm not sure that's related to my problem.

[CamGTP]

Hard to say if your truck has a real sensor for fuel pressure or not. I wouldn't know unless I looked for it on the fuel line. Vehicles with fuel pump control modules will vary the pressure depending on fuel demand. At idle and low throttle the fuel pressure will be around 3 bar (43.5psi) and will rise up to 4 bar (58-60psi) under heavy load. The performance cars will go even further to 65-75psi of fuel pressure because they need more fuel flow under boost.

 

 

[evad]

well.. if your engine is tuned up to spec and no other codes maybe it is an exhaust leak ...could the upstream o2 sense to much oxygen       making for a rich condition ? 

[davester]

It's possible for your broken exhaust manifold bolts to cause air to be drawn into the exhaust, fooling the O2 sensors into reading lean, so the pcm will dump fuel into the engine to try to balance it out.