2012 IS 250. So I have had the P0420 code for a few months now. I erase the code and it stays away for 4-5 days then pops back up again. I drive roughly 200 miles in those 4-5 days and probably 75% of it is city mileage.

Recently I drove from Tampa to Miami and then Miami back to Tampa. That's 600 miles total. The code never popped up. But a few days later after I got back and was driving in the city again, the code appeared. What's the reason for this??

 

What's the state of the intake and carbon build up? Has it been looked at or cleaned?

Steady state driving like the interstate has a lot less variation and could help clean up some carbon, and soot deposited in the cats. Do you have any long hills you can pull in 3rd gear so it works hard at RPM? Say, up in the 5k range? Cats do fill with soot and become less efficient if the engine is never opened up to kick the flow up.

You might try some Yamaha Ring Free product. Change the oil when done, assuming everything is healthy.

Lastly, does it consume oil?

 

Haven't had any issues with anything related to oil. I don't have many hills around me but I am sure I can find out. You're saying go up and down that hill staying in 3rd gear the entire time??

I ran a can of seafoam through the throttle body like 6 months ago. Immediately after that is when the code started popping up. I also use a bottle of STP complete fuel system cleaner about every 6-8 weeks.

 

Seafoam.......why do people use this 70 year old ancient stuff in modern cars? This is literally the worst fuel system treatment you can use on any modern vehicle with a three-way catalyst system. Modern three-way cats can't convert Seafoam vapors into CO nor into H2O. But Seafoam can kill cats.

Perfectly save to use in fuel systems which DON'T have catalytic converters.

Perfectly safe to use in engine oil.
Not safe to pour directly in the intake manifold of modern cars. Letting the engine suck up Seafoam directly into the intake manifold is a waste of $$$. That's not how the chemical works. Doing this will also reduce spark plug efficiency.

Replacing the sparkplugs might eliminate the P0420 code (city driving....better spark at low RPM while accelerating in stop-go traffic when more gas is flowing.

 

Basically we want the opposite of someone showing off their exhaust by doing static high revs in the driveway.
A condition that has it near maximum load, while at RPM and momentarily out of closed loop operation (running rich at 12.5:1 AFR) to light the cats off and burn off some of soot and ash so they perform efficiently again.

A pull from 1st to 4th isn't the worst thing (engine fully warmed up), no need to redline. Ideally a hill allowing the workload to go higher without driving at crazy speeds is best w/some wide open to get out of closed loop operation for fueling.
Make sense?

I run Techron 3X a year to get moisture out of the tank. So far all is well.
When was the last the last time you cleaned the MAF, inspected the intake?

 

I have a few bottles of Chevron Techron. I put it in the tank about 600 km before oil change.

IDK about other stuff.

 

P0420 happens when the car fails the OBD2 catalyst efficiency diagnostic that the car runs about 10 minutes after a cold start. Despite the 600 miles driven, you likely only had two opportunities to fail that diagnostic during your trip - one was 10 minutes after you started the car in Tampa to drive to Miami, and the other 10 minutes after you started it in Miami to drive back to Tampa. Once the diagnostic test runs, it doesn't run again until the next cold start, no matter how many miles you drive before then.

 

I pulled this off my Bluetooth OBDII.. I'm guessing this is bad..

 

That's good info! Where did it come from?

TIA!