2000 suburban 5.3 - P0300 P0420

My suburban has been having issues starting in the cold weather for the past few years and every once in awhile in the hot seasons. I constantly have been getting the 0420 error code even in the summer and as soon as I reset the code a day later it will come back. A few days ago it was having a heck of a time starting where it just kept turning over and didnt want to start. after a few hours of trying it here and there it started and stumbled for about 2 minutes like it wanted to die but finally started to smooth out and idled fine. Now it hasn't acted up for awhile. I have noticed at idle that the rpm fluctuates just a little but has never died. I tested the fuel pressure and it sits at around 60psi and when i shut it off it stays at 60 for about 10 minutes then it goes down slowly after that. I have also checked the pressure regulator and i don't have any fuel in the vacuum line.

Any suggestions?

2000 Chev Suburban 5.3L
120,000 miles
changed spark plugs about 10,000miles ago

P0300 Random/Multiple Cylinder Misfire Detected
You said that you changed the plugs. Make sure that you have the right type of plugs and the cables are seated right, sometimes they do not "click" in place. Possible damage on the cables when pulled to change the plugs. They do get sticky and sometimes they get a good jerk while doing maintenance. A good Spark Plug cable puller comes handy. I think the plugs on the rear left/right side is hard to get the cable off due to its position, specially when they get "glued" due to heat and grime for years of usage . With 120,000 miles I would advise to change the cables too.

Look on the wiring for possible damages or corrosion related problems.




P0420 Catalyst System Efficiency Below Threshold (Bank 1)
On this one I wait to fix the previous one, this one could happen as symptom from the previous one. Is possible, whatever is wrong with the ignition could fix this one too.

I have the EXACT same problem with a '00 Tahoe with the 5.3. I've replaced the plugs,and wires, cleaned the throttle body, checket the fuel pressure (60 psi.), and thrown tools at it. It's still there and very hard to start 75% of the time, and stumbles for a while.

Now a Plugged Cat can cause that too. I was working on a Grand Am and started loosing power, can not climb RPMs, Overheated more than normal.

It threw the codes for Random Misfires and EGR Problem. I changed the Cat and is good as new.

Why the Cat? back pressure causes similar symptoms. Gases can not scape therefore goes back to the Combustion Chambers killing the ignition and lowering the Temps on the exhaust to certain degree and the cat does not get hot enough. Also, the last sensor on the exhaust detects the Temp that goes out of the Cat, if is not hot enough, it will throw a code.

I said to do the previous thing first since is cheaper than replacing a Cat.

Cats do not fail often, what makes them fail is excess of Oil, Fuel and Glycol. That could be an indication of Oil Overfill, worn valve guides and anything that can add oil to combustion chambers. A good look at the plugs can tell, if they are tan is OK, if is black, wet, deposits is not. If you have a leaking intake manifold, Glycol can make it to the chamber, or blown head gasket can, but you will notice a blown head gasket.

It could be that is old, but I had cars over 150,000 miles and never changed the Cat at all.

One test that I read on the web is to drill a hole on the Exhaust pipe before the Cat so gases can scape, if it improves is a plugged Cat. If not, a self tapping screw can plug the hole.