P0420 just started popping up last week on my IS 250. The car runs fine still as if there isn't an issue but the code keeps coming back the next day after I clear it. What should I be looking for to fix this? I see the code is related to the catalytic converter

 

Check for exhaust leaks, especially the Y-pipe near and under the heat shield. Very common problem, especially in areas that use salt and other snow melting chemicals. (I see your profile says Florida, but I'd still check that first)

 

I couldnt see/hear anything when I looked. What else should I look for.

 

On the pipes, and flange mating from mid pipe to cat converter. Do you see any soot leaking out?

 

When moving it with my hand, no I dont. Should I check with the car running??

 

No it will be noticeable.

If you reset the code how fast does it trigger again? I had a failing cat converter for bank 1.

I went through all the trouble shooting stuff. In the end it was the cat. I reset code it was good for about month. Once it failed to a worse state. It would trigger more frequently. Until it would just trigger immediately after a reset.

 

Usually the next day it will reset. My commute every day is roughly 50-60 miles total. The weird thing is, there are no performance issues at all. Starts up right away. No rough idle, no stalling, no slow acceleration, getting a little over 25 mpg. It's a 2012 with 150k miles.

 

Mine was the same way. Car performance was the same the only annoying thing was the cel.

As far as performance the cat would have to be clogged up.

https://www.carparts.com/blog/p0420-...eshold-bank-1/

 

Sounds like it could be a bad O2 sensor. Old RX330 I had, had an O2 sensor go bad. Swapped with a Bosch one I got off Amazon for ~$75 or so. It'd be cheaper than replacing the cat if you're just trying to eliminate what it could be.

 

I have an 08 IS250 with 205k miles. I believe this happened to me around 120k and again around 150k miles. It hasnt happened to me since. All I did was remove the engine air filter and dust it off a little and reinstalled it. On both occasions the error code went away pretty quickly.

 

Code P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1).” Put simply, this code indicates that your car's computer has detected underperformance from the Bank 1 catalytic converter.