Thread: Persistent P0420. Please help--about to hock my 05 Prix for scrap I'm so mad.

These cars don't like aftermarket cats. I'd suggest having the code tuned out. Mine is tuned out. No MIL, no code reported.

Mine threw a 0420 code right after i installed my headers/hi-flow cat. Bought a PCM tune. Problem solved

Put a $6 spark plug non-fouler between the downstream/post cat O2 sensor and the exhaust pipe. That will increase the perceived catalyst efficiency by the ECU and poof, out goes the light permanently.

You have upstream/pre-cat and a downstream/post-cat O2 sensors. The upstream O2 sensor does 2 things. The first is monitor the exhaust gas so the ECU can adjust the air/fuel mixture. The other is the initial exhaust gas reading pre-cat. This reading is compared to the downstream/post cat O2 sensors reading. The difference between the 2 is the catalyst efficiency. The bigger the difference the better the catalyst is performing.

GM sets the P0420 CEL for a specific % efficiency. Newer years +- 90%. That means if the catalyst media loses 10% efficiency you get a CEL. Functionally that cat could go on for many years before it actually clogs. Some aftermarket cats have inferior media, others are about "flow" and performance. better flow means less time for the catalyst media to burn off gasses, less efficiency.

There are many DIY's on the net about how to mod the non-fouler and install. Its really simple. Remove rear O2. Drill out the tip of non-fouler with 1/2" bit. Install on O2. Screw both back into exhaust. 1/2 hour labor. Drive around for a day or so, out goes the light. I've had one on my Camry (Toyotas are plagued with unrealistic efficiency ECU programming) for over 100k.

Originally Posted by Basher

Already done, still get the light. That's what bought me 6 months before it started throwing P0420's again... so 'permanently' might want to be taken out of your repertoire bud.

Impossible if installed correctly, there would be no emissions passing the sensor. Can you be a little more descriptive on what exactly you did related to an non-fouler (what did you use, how was it installed, etc.)

Trying to help you and understand why its not working is all. I've done at least 20 of these, no ones ever come back or complained. If the hole is drilled out too much, or the non-fouler is too short leaving the sensor tip sticking out its only partially blocked. Did you install the second non-fouler un-drilled to cap it off?

Just tune out the code. I just passed emissions with my junk and it has no EVAP, no EGR, no cat. None of that bull****.
Everything read "READY" and no codes reported.

Originally Posted by Basher

I've been chasing a persistent Check Engine Light since I bought this damn car, 3 years old and 42k miles on it.

2005 GP Base. Nothing special. P0420, Catalyst Efficiency Below Threshold, Bank 1, bank 2 occasionally.

I bought a catted DP from ZZP. I did a resonator delete, and recessed the rear O2 sensor while I was in there. Aside from a properly prepped drop-in K&N filter, these are the only mods. All was well for 6+ months, now I'm getting the P0420/Bank 1 code again, persistently. Bank 2 is fine, so I know the recess job is working at least.

Seriously...I'm about fed up. I've spent $1000's in diagnostics and letting my mechanic tinker, as well as the parts themselves. Everything from new plugs/wires/coils, new MAF, new EGR, new PCV, 4 (yes FOUR!) sets of new AC Delco o2 sensors. In all I'm probably in for half of what I bought the car for, by now, chasing this one damned code. It would be cheaper to fire up the $400/month payments on a new car at this point.

I'm missing something, aren't I? I really need some help here. I love my Prix but ...I can't renew my registration with a MIL. I'm not particularly mechanically inclined (I can change my own fluids, brake pads, etc, but I don't have the place or tools to do the hardcore stuff) so any help is greatly appreciated guys.

thats your probelm. stock cats on these cars are right on the edge so anything aftermarket will set off the code, espically on newer models. Put a stock downpipe on and a good rear o2 sensor. you can also get the code tuned out but theres some other things that have to be done too so the car will run right, look at the "aftermarket cat on 02 plus vehicles" threard.

I have a ZZP catted downpipe with their "defouler" add on and I get no P0420 on my 07. The problem isn't the cat.

whats the defouler add on? you mean the emulator that supposedly doesnt work on 04+ vehicles? the cat isnt the problem your right but it is the problem, its high flow, so if your running a little rich it doesnt do what the stock cat would do and it sets the light. if he puts a stock dp on the code will go away and he can pass inspection.

this is from zzps site

***IMPORTANT information concerning High Flow Catalytic converters*******
The downpipes we sell come with a high performance converter. It is OBD2 compliant but it is on the very edge of being so. The PO 420 code is set when the rear bank O2 sensor PCM thinks that the converter is not doing a good enough job. Because the stock cat is so thick and dense, it can still function when the tune is off or it gets old. An error in tuning, running too rich, having too cold of a Tstat, too cold of plugs, or an old primary O2 sensor can all affect the engine enough to set a code when using the cat. This code DOES NOT affect the way the car runs, it just sets an SES light. To fix it, you'll need that code removed from the PCM, or you can run our O2 emulator(not available for ALL vehicles), or you can clear the code and try to tune the car in better to allow the cat to operate more efficiently. This is a situation in which your cars stock sensor was not designed with your performance goals in mind. Please make your purchase with the understanding that over time, an SES light could be thrown due to the reasons stated above. This is not grounds for a warranty claim, as a repair or replace of the cat/downpipe would not permanently fix the code.

Originally Posted by Basher

P0420, Catalyst Efficiency Below Threshold, Bank 1, bank 2 occasionally.

From what you've explained about the non-foulers agree with others, some other condition may be triggering the code. Above you state the P0420 is appearing on 2 different banks. Can somebody explain how thats possible?

Originally Posted by Swiggles

whats the defouler add on? you mean the emulator that supposedly doesnt work on 04+ vehicles? the cat isnt the problem your right but it is the problem, its high flow, so if your running a little rich it doesnt do what the stock cat would do and it sets the light. if he puts a stock dp on the code will go away and he can pass inspection.

this is from zzps site

***IMPORTANT information concerning High Flow Catalytic converters*******
The downpipes we sell come with a high performance converter. It is OBD2 compliant but it is on the very edge of being so. The PO 420 code is set when the rear bank O2 sensor PCM thinks that the converter is not doing a good enough job. Because the stock cat is so thick and dense, it can still function when the tune is off or it gets old. An error in tuning, running too rich, having too cold of a Tstat, too cold of plugs, or an old primary O2 sensor can all affect the engine enough to set a code when using the cat. This code DOES NOT affect the way the car runs, it just sets an SES light. To fix it, you'll need that code removed from the PCM, or you can run our O2 emulator(not available for ALL vehicles), or you can clear the code and try to tune the car in better to allow the cat to operate more efficiently. This is a situation in which your cars stock sensor was not designed with your performance goals in mind. Please make your purchase with the understanding that over time, an SES light could be thrown due to the reasons stated above. This is not grounds for a warranty claim, as a repair or replace of the cat/downpipe would not permanently fix the code.

I don't see the add-on listed as an option anymore, but when I got mine they had a "No SES" option. It basically added a "defouler", but it was a piece that was welded where the stock o2 goes and stuck out away from the downpipe.

I have the ZZP downpipe and I have an 07 and I have NO CEL. He has a ZZP DP and he has added a defouler. The cat is not why he has a CEL. He already stated the reason he went with the aftermarket downpipe is because he was getting a CEL with the factory cat.

Ah I misread and thought he got the car with the dp.