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.

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.

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.

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.

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.

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.

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-

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.

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.