99 p0303, p0420 need help!

[chitown906]

I've been chasing these codes for about 3months now , and it's really starting to get to me as I keep being told that if the p0303 is being thrown it could be my cat? But the p0420 only comes on every once in awhile .

As for trying to figure out what the issue is, I replaced the cap/rotor, about 500miles ago because the truck would not turn on, yesterday I went ahead and replaced all the plugs, and found that the plug from cyl 3 was fairly burnt up , pictures can be posted if need be , otherwise all the other plugs were fine. I also contacted summit to warranty my wires out so if that doesn't' correct the issue , I am more than just confused.

I've truthfully hit a wall , the only other ideas I personally have is replace the plugs, or replace the 02's , but outside of that I really dont know what more to do.

Any and all ideas would be greatly appreciated.

Chi_town

---------- Post added at 12:47 AM ---------- Previous post was at 12:41 AM ----------

From what I found on Google, these are the common issues , with a P0303

A code P0303 may mean that one or more of the following has happened:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer

p0420 -

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter

 

[SunlitComet]

a cat does not cause a misfire but a misfire can ruin a cat. Let me see pics of that plug and say #1 or #5 as well. and did you put delco 41-993 plugs in there now? What ever has gone wrong in #3 once the issue is fixed I am afraid your cat system may finally be fried.

 

[chitown906]

None of the parts stores had this plug in I went with Denso, I used them on my 94 Sierra and didn't have any issues do I'm hoping for the same results at least until I can fix the misfire issue .

 

[chitown906]

Cylinder 3 sorry it's really blurry

 

[chitown906]

Cylinder 5

 

[chitown906]

Cylinder # 1

 

[chitown906]

And, thanks for moving it wasn't sure if it would be considered the performance of the truck or just general .

Chi_Town

 

[SunlitComet]

Can't see crap. describe in detail please. And were the ones you put in platinum with a .060 gap?

 

[chitown906]

And yes they were , I plan on replacing them again after I do my intake gaskets .

 

[SunlitComet]

you either have an oil control problem thru the piston ring or valve seal or coolant is leaking in there. solve that and the ash deopits will stop. the deposit build-up what ever the cause is causing the cylinder to go dead ignition wise. I hate to say this but with that much crap on it you converter is fouled as well as the oxy's are most likely fouled a bit too.

 

[ande6009]

I have the greaat "mystery" P0303 as well, just found code after purchasing truck. Alledged "motor replaced with 50K mile motor recently." Seems to run fine, but nothing to compare it to. Plenty of power, guess ill be pullin #3 and see what lies beneath.

 

[chitown906]

Well mine has become P0300 which is from what I've gathered a vac leak from my intake manifold gasket. Which would also explain the plugs condition of cylinder #3. I'll be doing the intake job this weekend and HOPEFULLY that solves my new code , as well as the P0420 code I have gotten from time too time.

With your code just make sure you check all I've already checked and been told to check, try clearing the code and see if that helps.

Good luck

Chi_town