Code P0421 catalytic converter

P0421 is a converter that is below efficiency on a cold start. I would first verify that the O2 sensor downstream is functioning correctly, It could be sluggish or failing and reading that the converter is the bad part. If the sensor is reading good then it is likely a bad converter.
That car can have a single unit mounted under the car or two with one mounted to the engine and one underneath, it depends on the build date and emissions standard it meets. You will want to verify that before looking at parts. It also makes a difference as to the location you live in. In NY for instance, if the car is a federal emissions vehicle you have a lot more leeway than if it is a California emissions version. If you install the wrong one the fine is rather steep.
As for used units, they are illegal under the clean air act.
From the EPA - The EPA considers it a violation of the policy to install a used converter from a salvage yard or sell it for reuse unless it has been properly tested and labeled. Similarly, it is a violation to install an untested used converter brought in by a customer, even if the customer insists that the used converter came off his/her vehicle.

Thanks for the info, Steve. That makes sense why I couldn't find used catalytic converters then! Where might I find the emissions type and build date? Is that with the VIN and everything along the drivers side door well? And how do you test o2 sensors using a scantool?

Edit: I'm in Colorado BTW.

Emission type will be on the sticker under the hood that tells the plug gap, engine type and such. Build date is on the door with the VIN.

Colorado has a patchwork of testing requirements. If you are in Boulder, Broomfield, Denver, Douglas, Jefferson, or portions of Adams, Arapahoe, Larimer, or Weld counties you have to have an emissions test.

Check this map: https://www.colorado.gov/pacific/sites/default/files/Gas%20Program%20Map%20Area.pdf

If you are in those areas they will do a couple tests, the OBD II port and the dyno test with sniffer. They look for the monitors to be complete so erasing the code and taking it in won't work.
As to what you can install, I don't think Colorado has gone quite as far as other states but I'm not up to date on the latest rules out there. I know they were looking at adopting California rules on new cars and replacement converters a couple years ago but don't follow it. Hard enough keeping up with NY these days. Looking at RockAuto.com they don't show any restrictions as to C.A.R.B. only so you might be lucky.

Here's the info from the Vehicle Emission Control Information label under the hood. The picture is also attached. So what does this mean? What catalytic converter(s) would I need to buy? This is basically a permanent check engine light until it's fixed it seems.

Test Group 1DSXV02.4GNG
Evaporative Family: 1DSXR0165A1F
Emission Control System: EGR/HO2S (2)/WU TWC/TWC/SFI

The one I'm looking at is Eastern Catalytic® 70316 - Standard Universal Fit Oval Body Catalytic Converter. It says it isn't legal in CA, NY or MN, but I'm in CO. Also it says may require welding and cutting? Is this something any shop is capable of?

That is a California emissions vehicle sadly. That means to be fully legal it needs to have C.A.R.B. certified parts. Rock auto shows C.A.R.B. legal front units for about $450.00 and the rear for about $300.00 both plus shipping. What you might want to do is check with a local exhaust shop and see what they say, in NY and California you would have to use the C.A.R.B. parts, but I'm not 100 percent sure in Colorado as they keep changing things. If you are doing it yourself it might be possible to use the federal parts to lower the costs, but I would check there first. Wouldn't want you to replace them and then have them tell you the new parts are not legal in your area.

Is it possible they just have carbon build up and needs to be scraped out? My mechanic said that the code most likely means there's a crack somewhere though.

There is no scraping them out. Inside is a ceramic honeycomb coated with platinum and other metals in a wash coat. Those metals get burnt out over time. There are people who try to wash them out but I've never heard of it actually working on a high mileage vehicle, they simply wear out. As for a crack, while it is possible it usually doesn't set this efficiency code, it will cause a P0420 code which is a continuous efficiency code while the one you have it for initial start. I would however still test the O2 sensor first, a slow or failing sensor could set this code as it wouldn't read the output correctly.
The fuel trims look good so the first sensor is okay, but you would want to look at the rear sensors output on the first start of the day to see how long it takes to start sending a signal and if the signal is correct.

Thanks for the explanations. I looked up the symptoms for a failing catalytic converter (pasted below), which I have noticed none of. By the way; the check engine light actually turned off today, not sure why since I didn't clear them or disconnect the battery. I looked at the o2 sensor variables in my scan tool program, and it has o2 sensor 1 short term fuel trim and voltage, and o2 sensor 2 short term fuel trim and voltage. What should I expect to see for these on a cold start from when it starts to getting fully warmed up? I presume o2 sensor 1 is upstream and o2 sensor 2 is downstream.

Among the symptoms of a bad catalytic converter are:
Sluggish engine performance.
Reduced acceleration.
Dark exhaust smoke.
The smell of sulfur or rotten eggs from the exhaust.
Excessive heat under the vehicle.

Hello, I'm Danny.

My friend works R&D for MAGNAFLOW exhaust corp. They make a CA emission compliant aftermarket catalytic converter as a replacement.Here is the exact part number for your Eclipse including the CARB-california air resource board number if you're interested. Hope this helps and thanks for using 2CarPros.

In this case there likely are no symptoms. The reason being the converter seems to be working once it gets hot. The code you have says the converter isn't working properly only on the initial warm up, so for about the first 5 minutes or so it doesn't work then it starts working. The O2 items you would want to look at would be it's output voltage which should stay at a low level and not track with the front O2. The problem is that many scan tools will not show the readings fast enough to tell the full story. One thing you could try to see if it's just sluggish would be to take it out on a longish trip and drive it a bit harder than normal to get the converter good and hot. Then see what it does the next day. I would also try a new sensor before replacing the converter if you have any doubt about the readings. It would only take some soot or crud on the O2 sensor to make it sluggish when cold.

From a video I watched, it sounds like I want to rev the engine a few times and see that the upstream and downstream o2 sensors both switch from lean to rich and back with each rev(0 or .1 Volts to .9 Volts or so). They seemed to track exactly the same when operating correctly from the video. He also said the o2 sensors don't operate correctly until they're fully warmed up, and that they have their own heaters for this purpose.

Another video I saw showed the curves of the downstream o2 sensor were much slower (and wider) than the upstream o2 sensor, indicating it was bad. And a third video showed that the downstream sensor didn't fluctuate above.2 Volts or so, which was also bad.

So am I wanting to look at the deltas / waves from revving the engine a few times after a cold start? Or just observe both Voltages with it idling? You said "The O2 items you would want to look at would be it's output voltage which should stay at a low level and not track with the front O2." which I assume you mean while idling without revving.

What you don't want to see is what you described in the first paragraph. The upstream sensor should be switching rapidly if you rev up the engine, if the converter is working properly the downstream sensor should change to a close to steady voltage as the converter heats up and starts to burn off the exhaust gasses. In the image attached the left side images are the upstream and the right side are the downstream sensors. The upper set are a good converter while the lower set show a bad unit.
For the code you have, When the car is first started from a cold start the PCM starts a timer and it looks at the signals from the O2 sensors and if the downstream sensor doesn't start to smooth out fast enough (to indicate the converter is heating up and working) it sets the code you have. However because you are not getting the P0420 code the converter is working, it's just not working fast enough on the initial cold start. That is why you don't have the symptoms of a bad converter as well.
As for which unit to use, that is why I suggest talking to a local shop, Your car is a California emissions vehicle, using anything but a California approved replacement may not be legal. It also could cause you to have the same code you currently have because of the way the PCM monitors the emissions system your car has. Plus the unit you would be replacing isn't the one under the car, it is the one that is part of the exhaust manifold as shown in the second image. The upstream O2 sensor is in that unit while the downstream sensor is in the pipe in front of the second converter that is in the pipe under the car. The third image shows the under car pipe with the downstream O2 sensor location circled. That second converter isn't monitored at all. However you will want to check over the entire system for leaks, before replacement, it is possible that a leak at the front flange gasket where the front pipe connects to the center pipe could in theory set the code if it was able to leak until it got hot and sealed up. Would probably be worth it to swap out that gasket (circled wit G) with new one before you spend the money on the front converter. Replacing the rear converter in that pipe shouldn't be needed unless the car were to fail a tailpipe sniffer test with the front unit working.

I decided to look into fuel additives specifically for cleaning the catalytic converter. After a little digging, I found a Scotty Kilmer video recommend using 1 gallon of lacquer thinner added to a half full tank and driving 150 miles at higher speeds. Tons of comments said this worked for them. What do you think?

Hello again.

I wouldn't recommend using that method.Using lacquer thinner would just burn hotter and possibly melt or damage the converter from the higher heat. I've attached a picture below of the stuff I use in our fleet.You will have to order it online due to it not being carried by many U.S. stores. I use it on our diesel Mercedes Sprinters with great results to clean instead of replacing the $1,400.00 DPF. Hope this helps and thanks for using 2CarPros.

Danny-

Evidently products like Seafoam and Lucas Fuel Treatment are said to possibly help too, and have the best ratings on Amazon (4.8 for each). I can get a gallon of the Lucas stuff for around $25.00 and then treat the fuel system multiple times. Supposedly once the fuel system is clean, the catalytic converter can run hotter and burn itself clean. What would be the difference with a product specifically for the catalytic converter? I would say I like the idea better of doing multiple treatments over time, rather than a one and done bottle. I think that's probably why there are a lot of 1 star reviews of these additives, they are one and done for $25.00 and either work or don't immediately.

Clean or dirty fuel system makes no difference to the converter. It has a coating on the interior parts that acts as a catalyst which converts the exhaust gasses to something a little better. If the coating is worn out or covered with oil soot, getting it hotter just bakes it on even better. If it is worn out, replacement is the only option. Have you actually tested the sensor outputs yet? If not it is likely that nothing you do will make a difference if it's a lazy sensor that isn't heating up fast enough.

Alright ,I just ran the test. Here are five screenshots of the data comparison. They definitely don't line up perfectly.

Based on those the converter is barely working. Mitsubishi must have a wide latitude on the trigger for the code the way that looks. I'm surprised it also doesn't have a P0420 set due to the way it is responding even once it's warm.

Well, that was only a minute or two. Should I try again tomorrow and go longer? Not sure it was fully warm when I stopped.

That system should be working in about 45 seconds to a minute. You can see it starting to get warm in the third frame. What you don't want to see is that last frame.

It's weird because I don't appear to have any actual catalytic converter related symptoms, i.e. loss of power or black smoke. I guess I'll have Midas do further testing on it during my next oil change in 1,000 miles.

Alright, so I looked at the Liqui Moly product a little further. So you got to buy a chemical pressure sprayer too and it goes directly into the exhaust line, and burns up in the catalytic converter. This seems much more likely to succeed than something you'd pour into the gas tank.

Hello again.

Yes, it's very simple and effective. I've used it numerous times as I've stated.All you do is buy a cheap Hudson sprayer from home depot for $10.00. just remove the pre-catalytic converter Ho2 sensor and spray the contents into the exhaust pipe. I've attached a picture of a Hudson sprayer below.Hope this helps and keep us updated as what you decide to do.Thanks again for using 2CarPros.

Danny-

Usually you don't get the classic symptoms if the system has dual converters like yours. They only monitor the upstream unit on many vehicles like that. So the upstream unit starts to go bad, but the second unit still cleans up the exhaust. Plus from the way yours is acting it is still working a bit, just not enough to keep the sensors happy. I have seen many other failed converters that show none of the classic signs either, if the engine is running correctly and in good shape many times you see nothing. Usually the signs are really the issue that made the converter fail in the first place, like bad piston rings that were causing the engine to burn oil, but the smoke was cleaned up in the converter, or the bad injector that caused the converter to burn up and now you smell the rotten eggs and see the black smoke of a rich running engine once the converter finally fails.

I'm not losing much oil. I do know the valve cover gasket was leaking. But I think it may have sealed back up with the dried oil.

I bought a bottle of that Liqui Moly catalytic converter cleaner, waiting for it to be delivered. The fact it's sprayed directly into the exhaust stream makes it seem much more likely to succeed. Also since I'd have to replace two converters, I'm willing to try anything else first that isn't replacing them both.

You don't have to replace both. The front one is the only one that has sensors. The rear one just goes along for the ride. If you look at where the O2 sensors are sensor 1 is before the converter while sensor 2 is in front of the second unit in the pipe between them. If they did tailpipe sniffing then maybe replace it if you fail.

Hello again.

Well, it might be a long shot to try the liqui moly to help save you money as opposed to buying new catalytic converters. Yes, they can be quite expensive so I understand your dilemma. Keep us updated as to how things turn out.Thanks again for using 2CarPros.

Danny-

Yeah we shall see. Would you mind telling me exactly which hose I should be spraying it into?

Hello again.

All you will need to do is gain access to the exhaust pipe under your car. Just unscrew the pre-catalytic converter oxygen sensor and remove.Using a Hudson style sprayer or something comparable empty the liqui moly can contents directly into the exhaust pipe.Re-install the oxygen sensor. Directions are on the can. I believe you run the cars engine for 15 minutes to cycle through the catalytic converter.Hope this helps and thanks again for using 2CarPros.

Danny-

If this car has the dual units on it, which a California car came with, you don't need to go under the car to get to the pipe, the first O2 sensor should be under the hood in the exhaust manifold between the engine and the radiator. Then you would spray the product into the sensors bung then replace the sensor and start the engine. Do not forget to put the sensor in as the exhaust pressure could spray it all back out and cause a real mess. The downstream sensor is behind the converter being monitored in the pipe that leads to the second converter. This was a common thing on many vehicles. They would install a converter close to the engine. Then they would do testing and discover that those units were good enough for the federal requirements, but didn't do so well with the stricter sniffer testing in California so they added the second converter in the pipe farther back to finish scrubbing the gasses. On more modern vehicles they monitor all of the converters so a modern 4 cylinder can have the dual converter system but will have a bank one sensor one then a sensor 2 and a sensor 3 as well with 2 being between the two converters like yours and sensor 3 being in the pipe between the second converter and the muffler. Larger engines can be even more interesting with a couple that have 3 converters on each bank of cylinders.

I believe that exhaust manifold is behind a heavy duty heat shield. If you Google "LIQUI MOLY Catalytic-System Cleaner (#8931)", the first video that comes up appears to show it being sprayed into where the air intake hose connects to the engine. It says to spray it into the intake tract at regular intervals with the engine running between 2,000 and 3,000 RPM's, not to spray it all in at once first.

Hello,

Maybe look into a programmable chip for it and lean out the mixture. I don't believe there all that expensive now.

Tom

Hello again.

Yes, I watched the video. They just used a vacuum source to introduce the cleaner through the system with the engine running.They varied the amount and increased the engine rpm's to not stall the engine out. That way is just as effective. I personally introduce directly into the exhaust system.Hope this helps and thanks again for using 2CarPros.

I have the Liqui Moly product but still haven't tried running it through the car yet. In addition to the P0421 code, I now I have P0135 and P0442. I was told P0135 may be related to the catalytic converter as well, or just a bad O2 sensor. P0442 I've been chasing down for years though. First I replaced the gas cap with a third party one, and it came back. So then I tried an official Mitsubishi gas cap, and that didn't get rid of it either. From doing some digging this can be a very hard code to get rid of. Any ideas on these 2 new codes?

The P0135 is an O2 heater circuit code for sensor bank one. So it's the sensor before the converter that is giving the P0421. I would look at the wiring to it to be sure it isn't damaged or shorted. It won't be the reason for the 421 code though.

Could you discuss the possible sources of the small evap leak for 442?

We already concluded 421 is caused by one of the catalytic converters (my car has 2) since I actually did a live data reading with a scan tool and Danny told me the catalyst is definitely not performing correctly. But I have no symptoms of a clogged exhaust system, so it seems to imply the 2nd catalyst is taking care of the hydrocarbons. It's just that the o2 sensor only senses that the first one isn't working properly.

Hello again.

I will be with you in the morning when my references are working again properly. Thank you for your patience.

Danny-

Correct in that the P0421 is a bad bank one cat, the P0135 is the heater circuit for the first O2 sensor on that same converter. The two are only related in that they are the same bank.

As for the EVAP leak code, site policy is to keep different issues as separate questions to make site searches easier. So please use the link below to ask that as a different topic. Thank you.
https://www.2carpros.com/questions/new

Hello again.

I've attached picture diagnostic steps below relating to code P0135. As far as code P0421 that is a separate issue and will need to be addressed in a new question.

Here is the new question link:

https://www.2carpros.com/questions/new

Hope this helps and let us know if you have any further questions on this issue. Thanks again for using 2CarPros.

Danny-