So i've had this P420 code coming on and off now for a couple months. Haven't been able to put my finger on exactly what the issue is yet. Don't think its my gas as all i put in is Sunoco 93 shell93 and quick chek 93. Also not sure if its actually an O2 sensor as it used to give me a different code for another sensor. Also replaced gas cap as someone has said that could be an issue. Can anyone tell me what it could be? Don't wanna blow 200 bucks on an O2 sensor i don't need..

 

P0420 is for the rear catalytic converter, low efficiency.

I had the same code come up and filled up with different gas and it went away, I ended up purchasing some HFPCs to replace both cats.

If you're not getting the O2 sensor codes then your O2 sensors should be good. From reading around when I first got the code if you just get P0420 then your O2 sensors are working. There is one O2 sensor on the input to the cat and one on the output, so that's how it's checking the efficiency.

I was a bit concerned, since I read some that there's some chance the catalyst in the cat can break down and be sucked back into the head and cause havoc.

Didn't make sense to me though, since the exhaust flows towards the pipes, but I read quite a bit that it 'can' happen, somehow, if it ends up plugging your 3rd cat or something if the catalyst is actually breaking down. Seems it was being debated a bit in the threads I read.

Emissions mileage on them is 80k miles, or 150k miles if you're in CA so dealer would likely replace it if you're still under emissions mileage warranty.

Replacing cat yourself, if it is the problem, isn't the easiest task. SM calls for removing the j-pipe, passenger axle and intermediate shaft to get it out from the bottom. But people have been able to remove it without doing so by removing the heat shield first and then wiggling it out through the engine bay.

 

I've had this problem for over a year now..comes on and off..I still haven't figured out what to do..neither has my mechanic ..

 

That code usually indicates a failure in the Catalytic converter. Usually the first bank, which would be closest to the firewall. A proper diagnosis with an HDS can save you some money in the long run. Also, sometimes there is way to test them with a OBD diagnostic(usually for sensors), and i remember watching a video on checking the temperature of the suspected cat at the input and output flanges. An abnormal reading, or one not alike will confirm a bad cat.

 

hmmm ok so i guess i will have to purchase some HFPCS in the future but did that totally solve the issue? Does the light still come on for you mzilvar? thanks for input guys

 

My check engine light went away, I reset it once and it came back.

Bad fuel can cause this problem, so after I fueled up again I reset it again and its been gone since.

I was reading some about it there is a test with the HIM that can be done to actually test the cat efficiency and determine if it's faulty or not, pretty sure it's in the SM I'll see if I can find it again.

I haven't put the new cats in yet, they actually just arrived today and I have a notice from USPS to pick them up. I'll probably put them in this weekend with my passenger axle & intermediate shaft since I have new ones waiting for me to install as well.

I also purchased new O2 sensors, didn't need to and probably ran the cost up more than I should've since all new O2 sensors are like $300 but I figured I'd just install new ones in the HFPCs. Not really the best way to troubleshoot, but my O2 sensors are the originals and have about 101k miles on them now so I figured it's probably about time to replace them.

 

So it's actually the HDS, something most of us don't have access to.

Here are the scans for troubleshooting P0420 from the SM.

It was probably just bad fuel in my case, but oh well sounded like an excuse to put in HFPCs to me.

 

Any excuse to mod! Ha

 

Just fyi, my problem is gone. The fault was cleared before I put the HFPCs in though and now with them in with new O2 sensors it hasn't come back on at all.

My gas mileage did go up, I was averaging 18mpg on the streets here in Hawaii and now I'm averaging 22mpg.

 

Hey i just have a engine light for P0420 and just one code. So all you did was changing the 02 sensor?

Im looking at the CC but they RUN alot of $$$

 

I replaced all 4 O2 sensors, that was about $300 for new OE sensors and I put in RV6 V3 HFPCs, totaled around $1k for the sensors and HFPCs.. that wasn't really necessary though. I didn't feel like pulling the old O2 sensors out of my OE cats, they're actually still in them sitting in a box.

Also my code cleared after a week or two before I did any of that work, problem went away after filling with good fuel at a different station.

 

Have you guys try clearing the code with a scanner?

 

Ok thanks

 

I've had this problem for about 2 years and it's really Random and when CEL goes on or off...ppl have said to change the o2 sensors..but ppl have also said it may not work...I bough the car like this so I don't know if my mpg is being affected that much...

 

Check light came on yesterday and just clear the code.

I been pumping tower mart gas because it's the closest gas station but I will try pumping shell gas, since that's the next closest and see if the engine light come back on.

I'm driving on a 2005 RL with 120k

And before I do get 28MPG and now I been getting 22MPG

 

Just droppin some pennies. A Bluetooth ELM327 (~$20) + Torque Pro android app ($4.99) gives you the temps of the cats (both flanges as far as I can tell) and can perform O2 sensor checks (resistances, voltages, etc). At this point OBD2 scanners are obsolete. Glad your problem went away.