Help me troubleshoot my P0420......

[hirdlej]

Long story short, I fixed my scanner today and hooked it up to my 97 C1500 with the 4.3L. The truck keeps firing off a P0420 for inefficient cat. The truck has no misfires, runs great, no exhaust leaks but the post cat O2 is hovering around .0xx-.3xx then upwards of .9xx momentarily but never goes as fast as the other two 02's. That being said, does this sound like a bad sensor or if the converter was bad, would it cause the 02 to be lazy or make it appear to be lazy? Just wondering that's all. I don't want to switch 02's for the hell of it unless I know just because they're put near rusted in the pipes.

[hirdlej]

Ok, just got back from a test drive. The light is ON AGAIN. At idle it fluctuates between .150-.250MV, while driving it goes no lower than .500-.600mv. If I come to a stop it hangs at .500-.600mv and then as I idle it slowly comes back down to .150mv or thereabouts. Under WOT I saw .902mv. Someone please clarify and explain to me if the post converter 02 is supposed to act like a normal 02. Obviously it plays no part in fuel metering BUT should it be switching like a normal 02? If not, how should it be? If it is, then I have a bad 02. I'm going to try cleaning it in gasoline or some CDC Throttle Body cleaner tomorrow. I've got nothing to lose if the 02 is already bad. If I win, then yay me! This problem only popped up after a misfire developed in my truck (which is now fixed) I drove with the misfire for maybe 120 miles, I can't believe that would be enough to plug the converter (it's brand new magnaflow unit from february). At the same time the misfire occurred, I put 2 bottles of chevron techron synchron in the tank, I wonder if this plugged the 02 up with carbon or something. That's another thing to note anyways. Any info on this guys would be appreciated

[hirdlej]

I should also note that it's a magnaflow hi flow replacement......... That's what the exhaust shop put in. Another thing you guys should know is the original factory cat was replaced because it set the p0420 last winter. Upon further inspection the cat was broken up inside. (12 years old). I had them replace the cat and have been driving it fine for a few months with no issues. The distributor cap took a **** on me a few weeks ago while out of town and the truck was misfiring severely. Once home I replaced the cap and rotor and all is well now. I wouldn't think 120 miles would be enough to toast the converter. The interesting thing is although the truck was misfiring, it never set a multiple cylinder misfire code, just the p0420 (which I figure was due to the fact that there wasn't good combustion). Truck runs like a champ now I'm just fighting the p0420. I'm thinking tonight I'm going to remove the 02 (wish me luck as it looks like it's rusted into the pipe). Clean it off and add some spark plug non-foulers. I just want the light off as I know the truck is running fine. I'd drive with the light on BUT I don't want to not be notified of other potential problems so the light has to be fooled into going out.

[Mr. P.]

Well, it's either the cat or the sensor. My only suggestion is to replace the sensor with a known good working sensor from a 4.3L. This is the system with three O2 sensors, right? If you replace the O2 sensor and still see similar mV readings and sensor behavior then I would say that yeah the cat is fouled. It's a bummer you aren't local, I've got access to one here to try temporarily...

Mr. P.

[joecar]

The PCM tests the cats as follows:
- it waits for the trigger conditions to be present, and then...
- it first drives the AFR to stoich in closed loop, and then...
- it drives the AFR lean for a few seconds, then rich for a few seconds, and then back to stoich,
- then it watches for the front O2 sensor voltage to transition,
- then it watches for the rear O2 voltage to transition transition in like manner,
- it measures the time from front O2 transition to rear O2 transition,
- if the time is too short it sets P0420 (bank 1) or P0430 (bank 2).

So, if the front and/or rear O2's are not working, then either the test won't be run (if O2 fails to allow stoich or CL) or the O2's fail to transition in response to the commanded AFR... remember the test fails only if the transition delta time is too short.

[Gh0st]

just turn it off

[joecar]

That's what I did.

[hirdlej]

You tell me how to turn it off and I will........ With my HPTooners (pun intended) I don't have access to my 97 vortec 4.3l PCM.

[hirdlej]

Well, it's either the cat or the sensor. My only suggestion is to replace the sensor with a known good working sensor from a 4.3L. This is the system with three O2 sensors, right? If you replace the O2 sensor and still see similar mV readings and sensor behavior then I would say that yeah the cat is fouled. It's a bummer you aren't local, I've got access to one here to try temporarily...

Mr. P.

Mr. P. this is kind of what I was thinking. A few things I was going to try...
Remove the oxygen sensor and clean it with some gasoline or cdc throttle body cleaner then reinstall.
At the same time, reinstall the oxygen sensor with spark plug non-foulers to keep the o2 sensor out of the exhaust stream (make it think it's working)
Then lastly if that doesn't work, instead of replacing the oxygen sensor, I'll just put a o2 simulator in it's place and plug off the post cat 02 bung


One more thing for you guys, I drive this work truck like an ABSOLUTE GRANDPA. It never sees more than 2K RPM's hardly and spends a lot of time putting around town with a highway trip maybe once every 2 weeks. With the 3.42 gears it's always chugging around. Another site I read today mentioned that aftermarket cats (which I have a magnaflow) plug up easier than a fatory cat and run cooler as well. A write up suggested cranking the AC, rolling the windows down, turn the headlights on, radio on, fans on high, and run every accessory you have at once to get the engine working hard. Then they said go flog it on the highway for an hour to get the converter piping hot and if you're lucky you can clear it out. I don't know if it's a fluke or not but after resetting the light last night and hot rodding the poor work truck on the interstate for an hour today I still haven't seen the light come back on. Maybe I got lucky? Who knows. Time will tell.....

[Mr. P.]

... A write up suggested cranking the AC, rolling the windows down, turn the headlights on, radio on, fans on high, and run every accessory you have at once to get the engine working hard...
ROFL uh hmmm lesseee so idling the engine with the A/C compressor and alternator on (15-hp worth of effort) make the motor work hard?! GTF outta here... :headshot[ Wait - I take that back, this statement might well be true for a Yaris or old Civic :notooth[1

I have no idea if the Magnaflow cats have a harder time lighting off. But I will chastise you on this, since you're not demanding the extra performance why didn't you put a stock replacement cat in there? Hindsight... I would consider taking the truck (with the MIL on) back to the exhaust shop and tell them they installed a defective cat and demand they replace it with a factory cat.

AFA driving conservatively, if the motor is set-up for it [driving conservatively] then I don't see the issue; GM has spent many millions of $$$ in subtle refinements to get the vehicle up to temperature as quickly as possible to meet emissions. So as long as you have the 195* thermostat and factory emissions in working order + factory tune then I don't see how you can be running the motor/exhaust too cold.
Mr. P.

[hirdlej]

Truck is Bone @$$ Stock. it's a 4.3L 5 speed truck so I don't exactly hot rod it around. I'm going to see if the light comes back on and if it does I'll go to the exhaust shop and make them put an OE replacement cat in. I didn't ask them to put the magnaflow in there. God knows this truck couldn't be fast if you pushed it off a cliff. I wasn't attempting to put any more power out with the magnaflow cat. I said I wanted it done cheap and that's what they put in. It was $150 and some change out the door.