P0420, stock exhaust with muffler delete
 

I've had my FRS for 2 years on the 18th, and have had a muffler delete from a local exhaust shop for all but a few weeks of that time. About 6-7 months ago, I took it in for the TSB related to the rpm fluctuation at idle, and the dealership performed the ECU reflash. I've had the rough idle/prolonged cranking etc issue since, but I have never thrown any CELs, until last Sunday June 8th.

I took it to a local Toyota dealer and they told me it was a P0420 (catalytic converter efficiency below threshold), and they had some lean/rich issues and a rough idle. They recommended I take it to the dealership I purchased it at, which was slightly further away, and check with them since they would be more familiar with problems related to the car. My CEL went out on my way to that dealership (and hasn't returned), and when they pulled the codes, they verified the P0420, and told me they were ordering a new catalytic converter (although personally I would've checked the O2 sensors first). I have my appointment for replacement/repair Monday, but I can't help but wonder what caused the issue to begin with.

My speculation is that, due to the rough idle, it caused a buildup of unburnt fuel. This in turn, damaged the catalyst over time. The car still drives normally, isn't down on power and isn't louder or anything. I'm going to have them check into or schedule an appointment for the rough idle/prolonged start TSB while I am there because, although I haven't thrown a P0016-0019, the rough idle could be degrading the catalyst similar to a prolonged misfire or bad spark plug. Just a bit of forewaring and info for people.

My only "engine mods" are an aFe drop in, sound generator tube delete, oil catch can and the muffler delete. I've left the engine stock for warranty purposes and because I plan to go turbo eventually (don't want to waste $ on a CAI or header I can't use).

They wound up replacing the header/catalytic converter, and I asked them to look into the rough idle and there was no result of that. I'm still curious as to why the catalyst would deteriorate at such a low mileage. Oh well.

Defective CAT maybe? As least it's on record you mentioned the idle issues and the Cat shouldn't be deteriorating that quickly.

Their are 2 cats in the exhaust the main secondary cat, big one in mid pipe, and the primary cat in the headers.

As far as i know the promary cat is monitor by the two 02 sensors either side of the primary cat in the headers.
The P0420 code is usually thrown up when you go to catles headers or you have a exhaust or gasket leak near 02 sensors, or the primary cat is stuffed.

If the O2 was stuffed it should thow a related code and engin would generally run badly

You can usually pick a exhaust leak by staining near leak or condensation dripping on cold start.

The main cat is usually too far from 02 sensors to be monitored effectively.