I just wanted to give a suggestion regarding the same problem we've all had at one time or another. I bought and 06 3.9 that constantly had the nuisance code p0420 often caused by a missfire. Not a hard miss just a slight bump in the seat once in a while at a stop light. The previous owner had obviously tried the coil/ plug replacement technique because there were 8 different brands of coils on the car and some rediculous autolite ir plugs. I just couldn't justify spending hundreds on coils to maybe fix the problem so I did a little investigating. What I found was a slight air leak where the pcv inlet (the tube between the valve cover and air intake) connected to the intake tube. The plastic retainer was cracked so it was not sealing tightly. Since this is after the MAF surely any unmetered air entering at this point will effect driveability, especially at idle. After addressing this issue the engine already idled much better although I had a slight whistling sound still occurring. My experience with plastic intake plenums is that the gaskets/ o-rings dry out, become rock hard, and are so brittle they crumble the second you look at them. This also applies to you v6 guys because the 3.0s are notorious for this issue. Sure enough I replaced my old crumbly gaskets with an 8 dollar set of fel-pros from RockAuto and the thing purred like a kitten even with God knows what kind of ignition components. I've since replaced the plugs with OE but still have the mix and match coils and still no p0420! Just another thing to think about since I see a lot of people replacing their whole ignition system and have little if any results. Hopefully this will help relive somebody's migraine.
Try this before replacing coils! Intake gaskets! P0420
- Thread starter Zach2211
- Start date
RigsLS
Dedicated LVC Member
- Joined
- Mar 10, 2012
- Messages
- 7,756
- Reaction score
- 532
Where's my drinking glass?
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
This is a DTC stored due to a CAT slightly below threshold, meaning the efficiency of the catalytic converter and/or O2 sensor is not so great. A P0420 code will cause no symptoms aside from the MIL (malfunction indicator light) will illuminate.
Honestly replacing a gasket at the inlet in the system is not going to resolve a sensor telling you your CAT is not as efficient as it possibly should be.
"lot of people replacing their whole ignition system and have little if any results."
^ This is a false statement! It simply is ... no need for an explanation, it just is. Sure there may have been the odd fella that still had some misfiring issues after he replaced all coils and plugs but we've always learned that either a) They did not replace all or even with OEM, b) They didn't gap correctly or c) Had further underlying issues like wiring, connector or PCM.
Now stepping back and looking at the drawing board we can clearly come to realize that both P0420 & P0430 are directly related to the reporting of inefficient catalytic converters which at one point got fouled by running with a previous misfire for too long before correcting the real issue. That issue being "misfiring" of the ignition system. Raw fuel got dumped downstream and the CAT's didn't like it.
Simply replacing gaskets at the intake to prevent unmetered air from entering is not going to resolve any downstream inefficient readings of a catalytic converter.
At best it would correct a "too lean" or "too rich" DTC which is not related to anything downstream, it's more related to pre-CAT also known as the upstream O2 readings.
Now, granted you clean up the front end, the rear end will be much happier as a result. You WOT that LS enough with that somewhat inefficient catalytic converter on BANK 1, it will once again store that same DTC, just a matter of time. Same goes for the opposite BANK 2 which results in a P0430 stored. It will most likely store said code within a week or two depending on how hard you ride it.
Don't take my word for it though, I have no clue about anything, I only speak of my own experiences. ~ CHEERS
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
This is a DTC stored due to a CAT slightly below threshold, meaning the efficiency of the catalytic converter and/or O2 sensor is not so great. A P0420 code will cause no symptoms aside from the MIL (malfunction indicator light) will illuminate.
Honestly replacing a gasket at the inlet in the system is not going to resolve a sensor telling you your CAT is not as efficient as it possibly should be.
"lot of people replacing their whole ignition system and have little if any results."
^ This is a false statement! It simply is ... no need for an explanation, it just is. Sure there may have been the odd fella that still had some misfiring issues after he replaced all coils and plugs but we've always learned that either a) They did not replace all or even with OEM, b) They didn't gap correctly or c) Had further underlying issues like wiring, connector or PCM.
Now stepping back and looking at the drawing board we can clearly come to realize that both P0420 & P0430 are directly related to the reporting of inefficient catalytic converters which at one point got fouled by running with a previous misfire for too long before correcting the real issue. That issue being "misfiring" of the ignition system. Raw fuel got dumped downstream and the CAT's didn't like it.
Simply replacing gaskets at the intake to prevent unmetered air from entering is not going to resolve any downstream inefficient readings of a catalytic converter.
At best it would correct a "too lean" or "too rich" DTC which is not related to anything downstream, it's more related to pre-CAT also known as the upstream O2 readings.
Now, granted you clean up the front end, the rear end will be much happier as a result. You WOT that LS enough with that somewhat inefficient catalytic converter on BANK 1, it will once again store that same DTC, just a matter of time. Same goes for the opposite BANK 2 which results in a P0430 stored. It will most likely store said code within a week or two depending on how hard you ride it.
Don't take my word for it though, I have no clue about anything, I only speak of my own experiences. ~ CHEERS
I'm just saying it's worth a try. We know all cars and repairs are unique in their own way. The cat is not gonna be able to operate as efficiently as possible due to the raw fuel being dumped into it from a misfire. It's not built to handle that level of hydrocarbons hence the code. Now if I had continued to dump fuel and burnt it up it would be the cat's fault but it takes a while to get to that point. If I had just replaced the cat I would just melt down the next one. The DTC was not my problem. It was just annoying. The missfire was my problem and I cleared that up. It was also the reason for the DTC. DTCs do not cause problems obviously, they react to existing problems to let you know you need to take care of them. I've seen this fix work before so just thought I'd throw it out there.
Appreciate the idea Zach. I'm a little unclear on which gaskets you replaced. Could u elaborate please? THanks.