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I recently (within the last 3 weeks) purchased a used 2011 CR-Z with 124K miles, and after driving it for about 800-900 miles the CEL came on with the P0420 code. I'm a bit concerned that this code came up so quickly after buying the car. The car had one owner and according to its carfax had been regularly maintained at a dealership.

I don't believe the car has issues running when the CEL is off, but then again I don't have a lot of experience driving this specific car, so I'm just kinda assuming it'ss running fine. Of course, with the CEL it goes into limp mode, but even then my MPG still seems solid around 40-41mpg as I've read around that's a factor to look out for.

I plan on taking it to the dealership but I'd like to first try cleaning the cat with BG cf5 on a low tank, and then add BG 44k and see what happens. Thoughts are appreciated.
 

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I know there's a couple of other threads on the forum, talking about the same P0420 DTC, but I can't find them in my search right now, but if you have other DTC's along with the P0420, you need to troubleshoot those particular codes first. So, are there any other DTC's displaying too?

I don't know about the BG 44k or other fuel system cleaners, but for $20, it's worth a shot, a guess. Also heard of people adding a gallon of lacquer thinner to a 1/2 tank of gas to "fix" the problem, but I'd be afraid of creating more problems to the fuel system than you hope to fix, e.g., fuel pump and seals (so I'd rather pay for a new cat than take the chance.)
 

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So I have had the annoying p0420 code the past few months. It will come on, then off, then on, then off. No other codes.

I have read multiple threads on the topic here, but other than someone selling someone a header, there hasn't really been anyone who has said they have resolved, or been successful at fixing.

This past weekend I went ahead and changed the Spark Plugs, and did my valve adjustment (was due anyway) - car runs so much smoother.

But this morning on the way to work I got the code again. My car isnt really sputtering, or running in "limp" mode as some people have suggested with this code (accelerates fine, still getting mid to high 30's MPG, etc)... I just dont like orange lights.

So my understanding is the the p0420 code gets kicked off when the primary o2, and secondary o2 numbers aren't jiving - typically meaning the CAT is clogged, or not performing correctly not getting the correct exhaust combination between the 1st and 2nd o2.

What I am looking for is a way to diagnose the o2 Sensor instead of just replacing. I dont mind blindly throwing money at parts, but when the cat is already 500+ I dont want to spend 250 at throwing o2 sensors if that isnt going to fix it. I am pretty well mechanically inclined, so looking for tips before breaking down going to a dealer to get a 200 diag fee.

Since I am not having any other symptoms of a bad cat - I feel pretty confident that it is just an o2 sensor just wanting to narrow down which one it may be? I already checked the wiring and no issues there. I would think it could be the primary o2 starting to go bad sending a weak signal, so the car computer thinks the 2nd o2 has a higher reading?

Anyone else had any luck in resolving this code themselves?
 

· Modified 2016
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P0420 indicates that the CAT isn't filtering out pollutants efficiently. The downstream O2 sensor reads the exhaust gases from the CAT. If the values are below a minimum threshold then the car throws the code. In your case you may be barely below the threshold but not enough to notice a performance loss.usually the fix for this code is a new CAT. You also need to find out what caused the CAT to fail or the problem may return with a new CAT.
 

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Instead of listening to us go download or obtain the FSM (Factory Service Manual) and follow the trouble shooting process for that code. Code does not equal bad part it tells you which system and which process to follow.
So I have had the annoying p0420 code the past few months. It will come on, then off, then on, then off. No other codes.

I have read multiple threads on the topic here, but other than someone selling someone a header, there hasn't really been anyone who has said they have resolved, or been successful at fixing.

This past weekend I went ahead and changed the Spark Plugs, and did my valve adjustment (was due anyway) - car runs so much smoother.

But this morning on the way to work I got the code again. My car isnt really sputtering, or running in "limp" mode as some people have suggested with this code (accelerates fine, still getting mid to high 30's MPG, etc)... I just dont like orange lights.

So my understanding is the the p0420 code gets kicked off when the primary o2, and secondary o2 numbers aren't jiving - typically meaning the CAT is clogged, or not performing correctly not getting the correct exhaust combination between the 1st and 2nd o2.

What I am looking for is a way to diagnose the o2 Sensor instead of just replacing. I dont mind blindly throwing money at parts, but when the cat is already 500+ I dont want to spend 250 at throwing o2 sensors if that isnt going to fix it. I am pretty well mechanically inclined, so looking for tips before breaking down going to a dealer to get a 200 diag fee.

Since I am not having any other symptoms of a bad cat - I feel pretty confident that it is just an o2 sensor just wanting to narrow down which one it may be? I already checked the wiring and no issues there. I would think it could be the primary o2 starting to go bad sending a weak signal, so the car computer thinks the 2nd o2 has a higher reading?

Anyone else had any luck in resolving this code themselves?
 

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So took it to the dealership and they couldnt pinpoint anything. The code has gone back to coming on/off randomly. They said it could be the cat truly is clogged, or it could be the o2 sensor intermittently failing although they didnt see an issue during the hour they had it.

So I just took a risk and bought a used CAT from ebay out of a 2016 CRZ with 12,000 miles that also has the o2 Sensor for 220... I am hoping it resolves the issue.
 

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used to get it intermittantly all the time. started using premium gas and that helped, but didnt eliminate the p0420 entirely. I don't get why CR-Z O2 sensors are like 2 to 3X more spendy than other cars?

I saw a utube about seaform cleaning cats. take out the 02 sensor and spray seafoam directly into the cat. put the o2 sensor back and run it. i have not tried it. seems a little risky and my alarm prob isnt that big a deal. To me anyway.

-jim
 

· Modified 2016
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To anyone unfortunate to fall pray to the dreaded P0420 code I feel sorry for you. Just a quick search on many Honda parts sites shows that the catted downpipe is about $1042 and the catted header is even more expensive at $1400. Thats even more expensive then an OEM catted downpipe for my Mustang ($1050). That is the reason I did not junk the OEM catted downpipe on my CR-Z yet.
 
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My car has had the same issue of being in limp mode for the past year. It’s giving me P0420 and P0113 codes. It was giving me a bad mass airflow sensor code but stopped after changing it. Limp mode went away after replacing the MAF sensor but came back after a couple of weeks. Does anyone know the exact part numbers to the upstream and downstream o2 sensors? I have a 2011 ex cvt. I’m replacing the spark plugs in a few days since my engine is at 190k miles and got it at 105k. Doing a cvt fluid change & filter as well.
 

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I see some aftermarket solutions out there, like this one from HKS. It's more cost-effective than buying both OEM parts, which I imagine I'll have to do eventually since I intend on keeping this car until it's good and dead.


Anybody out there have good or bad experiences with aftermarket converters, or am I better off getting an OEM piece?
Did you end up buying the hks cat
 

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Is it possible your O2 sensors are disconnected or damaged? They measure your Catalytic converter efficiency. The usual mouse milk cleansers are not going to fix that( and as you saw can make it worse) . since it has had the issue since you bought it you have no idea what kinds of dumb things the previous owner did to the car. Bad fuel wrong fuel other abuse that could clog the catalytic converters. Or your converters may have simply failed, if it had been taken care of sooner then it may have been handled under the warranty which for emissions is longer than the other warranties. Or even under the power train warranty. Once you start making changes like you have though you run into issues of trying to prove your changes ( manifold) did not cause the issue. the Magnuson–Moss Warranty Act should protect you from that though.


My suggestion would be to follow the trouble shooting process for this code from the FSM ( Factory Service manual) or All Data or any number of other sources. There is a link on this forum to the FSM or you can buy one here http://www.helminc.com/helm/Result.asp?Style=helm&Mfg=AHM&Make=AHM&Model=CRZ+&Year=2011&Category=&Keyword=&Module=&selected_media=

Where is the threshold bank 1 o2 sensor???
 

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spdbump I know its not from the changes I made because the light started being permanent before those. reading up from the service man and doing some research, I have a good Idea that ive destroyed the materials in at least one of the cats if not both (I drive the car very spiritedly for 2 years since the light first appeared and still do I also use it for delivery work.) I do all the wrench turning that I can when it comes to repairs)
in trying to make the car faster. theres a lot I want to share and maybe ill start a thread of all the issues ive come across one tht might help this thread is my z struggles after passing 100mph versus before the constant cel I hit 136mph

theres still more that I WANT to do (saving up for) my next project is working on the exhaust line anyways. and currently here are my thoughts bare with me;

1# I started removing a 2009 honda fit main cat converter from its mani to replace mine.it was difficult without seeing and doing my hw so I gave up but I know for a fact they are identical to the z (still need help)
2# I want to delete the main and secondary cats possibly by using ge8 aftermarket manifold downpipe and a midpipe then figure out a way to disable or trick the cel. (fake main cat for inspection purposes or aftermarket)
3# I love the way my z sounds now so if feasible I will try to keep the resonator and muffler
4# I might start uploading videos on youtube just for every change I have and will do to my z. got a lot of personalization ideas inside and out that hopefully include an engine swap if not then a turbo
You still got the z?
 

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Hey guys,

2012 / 242,000 km / Bone stock everything.

seen some chatter on this issue over the last few years and Ive read up on all the forums that contained anything related to this.

Unfortunately, I've received the dreaded p0420 code (catalytic limits reached) and the car goes into limp mode.

Here's the thing - it started happening last spring, then it went away all summer. Now, (in Toronto) we are starting to get some cooler weather again. So, its back.

Mechanic says its the catalytic (one of them). But, if this was failing would it not at all, AT LEAST ONCE, have thrown the code over the last 3-4 months? It is definitely temperature related and it would make sense that if the cat is cold, isnt as effective and then throws the code when the cold cat isnt burning up everything it should.

My question to those that have been through this - was it in fact the converter ? Did replacing it actually solve the problem ? Or, was there another culprit in the mix ?
 

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A trick that is sometimes used is the Amazon.com: CRC Guaranteed to Pass Emissions Test Formula: Sports & Outdoors products which supposedly clear the CATS. it is worth a shot. Other have replaced O2 sensors, replace one or the other CAT though :( If the clean works it might be just enough to bring back cold weather performance but I am speculating wildly as I have not had that code yet. I have a 2013 though.
 

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Like you I researched P0420 on the forum when I got the code a couple years ago (about 120k miles at the time). I saw that for many it was their primary cat that died. I took a lucky guess and replaced the cheaper secondary O2 sensor first, and that fixed it for me. I am at 149k miles today. 2011
 
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