Kael

Hey all, still chasing the P0300. Saw something somewhere (heh) that mentioned a P0300 caused by a bad cat. I ran through testing my cats and lo and behold, one of them is Hot in, Cold out (that's bad, fyi, it's supposed to heat the air with the chemical reaction).

So, I'm under spraying PB on this and that to prepare for cat removal and notice two O2 sensors, one in and one out on the cat tube.
Shouldn't these have caught this problem and thrown a different code?

kevinkpk

PO300 is misfire. If the O2 sensors are bad, you would have gotten a code for that.These are for emissions.

oldchevy

I have always heard that a misfire in a cylinder that is left unrepaired for a while can cause a cat to get clogged up causing poor performance. This will give you a P0300 as the cylinders in line with the bad cat randomly misfire because of the back pressure.

mountainmanjoe

Isn't it the other way around? A misfiring cylinder is passing unburned fuel into the exhaust stream, which kills the cat.

oldchevy

Isn't that what I said?

mountainmanjoe

You said the misfire causes clogged cat which causes misfire.

So which came first? The chicken or the egg?

oldchevy

I said the misfire causes clogged cat which gives you a PO300 code which means there is a random misfire.

tech2

a cat that isn't redox reacting won't cause a misfire and would set a po420 p0430 low efficiency dtc.
as stated, the only way a cat causes a misfire is by creating an exhaust restriction. exhaust restriction can set misfire codes. if you have a scan tool that shows misfire counts; use it to determine which cylinders are accumulating misfires. a clogged cat would misfire the entire bank of cylinders. a clogged cat would also cause a lack of power.

MitchM

"a cat that isn't redox reacting won't cause a misfire and would set a po420 p0430 low efficiency dtc.
as stated, the only way a cat causes a misfire is by creating an exhaust restriction. exhaust restriction can set misfire codes. if you have a scan tool that shows misfire counts; use it to determine which cylinders are accumulating misfires. a clogged cat would misfire the entire bank of cylinders. a clogged cat would also cause a lack of power. " Thanks very much for this advice . how do I get my chevy dealer to 'use its scan tool to count which cylinders are misfiring' --to see whether this cat converter clog is possibly causing the p0300 that i've had for 9 months and spent $1500 so far with 'fixes' that don't fix it ? I drive the van in, hot and coding MIL steady then flashing then steady, with a DTC p300 . it sits on their lot for 2 weeks, they tell me it's a cam position sensor as codes p0300 and p0334 are stored, they replace the CPS . $500 later that still doesn't fix the problem. as a GM owner since my dad bought his first chevy impala , I am really disgusted w/ chevy's lack of diagnostic capabilities . ( the dealer won't even look at my actron's live scan and freeze frame data. )

Kael

New cat changed nothing, p0300 pops as usual (higher speed and no change in running engine). Odd the bad cat or O2 sensors never triggered a code. *shurg*

Ready to take the $200 plunge and have a shop check the crank sensor and relearn. This van did have 300k when I bought it.