Yes, this is another thread about P0420 code But mine is completely different from what I have read in this forum .

The car is a 2009 lancer es, everything is stock except k&n intake, and the cat has been changed to magnaflow universal welded one. The car is sitting at 183k miles (yes I drove a lot), and I changed spark plugs at about 105k miles ago, and recently had seals replaced on the transmission at 150k miles. The O2 sensor was also new, just recently installed in december or so last year, NTK series.

I keep getting the P0420 cat efficiency issue code, went to the dealer they told me it's the cat because it's to small in size. I called magnaflow up and they argued that it's not the cat size that matters because they used a better catalyst than OEM's, hence that's why it's smaller. The light comes on and off though, which is the weird part. It comes on randomly if I drive like 40 minute at 90+F degree weather, and if it's 65-75F weather, the light NEVER EVER comes on. However, last night it was 91F and the light didn't come on at all when I'm driving.

Mitsubishi scanned and couldn't tell me what issue I'm having, but that scan was "free" due to a recall belt replacement. They want to charge me $105 to do a "thorough" diagnostic, but I am afraid they will take more than an hour, and next thing I know I am about $500 out of pocket and issue not resolve.

From physically looking at the cat, it does look like the cat is shot. The color is brown/bronze, and it's purple/blue at the ends! However, WHAT is causing it to go out, is the problem I'm facing at the moment. I need all the advise I can get please. I just bought the direct fit bolt on to replace the welded on from magnaflow, but if it goes out again, I will not have money to keep replacing parts.

 

Try a steel washer (shim) to pull the O2 sensor out of the air stream a wee bit.

Stack another on (if you can/have enough threads) if there is still an issue.

Then go to a "non-fouler" solution.

 

Thanks Veronica for the reply. I believe this is told by me from a friend before, but this would only be a work-around, and "cheat sheet" for the car to not pop on a P0420 right? I'm trying to dig deeper into finding out WHY my Cat is blown so fast. Any other suggestion that I can look into?

 

Another way to look at it is:
They won't replace the mini-cat.
You won't (I presume) put the stock one back on

so... the 'cheat fix' (washer) may still be the best solution overall.

 

So I decided to change the Cat to Catco instead of magnaflow bolt on. Installer is certified emission and cat specialist, and magnaflow dealer but he told me magnaflow cat is made for performance only, not the low end daily driver like what I got. It will throw CELs all day long for the O2.

Apparently also he told me something is making the Cat heat up really high, 500F because the old magnaflow turned purple/blue. So far the new Catco is living up to it's name. I have had to CEL, got my gas mileage to about 26-27mpg.

 

Good deal!

The last one probably did have an internal issue - and should be returned to the OEM.
The high heat and poor mileage indicate it was plugged up...

 

Been driving for about a week or so now, the Cat is holding up quite well and I got the dealer to actually upgrade my software on the ECM for free instead of charging me $60. Apparently there was a TSB for a P0420 that keeps popping up due to software issue, but it was never addressed.

I did noticed my gas mileage decreased a little to now average of 25-26 after the upgrade of the ECM software. Is there a way to help diagnose if I have a leak somewhere in the fuel line or any hose leaks?