TL;DR: O2 sensor data, thermal measurements, and physical resonance tests all point to a failed Catalytic converter.
So, I replaced the gas cap, drive cycled, and MIL illuminated with same codes. I visually inspected system (lines, manifold, etc). Nothing obvious.
Took it into two shops: a dealer and an independent. The dealer's plan was to replace the catalytic converter. The independent's plan was to replace the carbon canister. After some research, agreed that replacing the canister was the best response.
Replaced the carbon canister, drive cycled, and MIL illuminated with only P0420. Progress.
- P0420 code (read 3/3), P0440 (read 3/12), P0446 (read 3/15)
- faint fuel smell while at rest and after driving
- gentle rattling/pinging on acceleration
- depressed fuel efficiency (two year avg: 32.4mpg, four month avg: 28.2mpg)
- decreased acceleration power (perception, no hard metrics)
I am no expert. This is my first experience with the P04xx series codes. I'm reading a bunch online and going into two shops for expert advice.
The dealer and independent both suggest replacing the cat at this point. That's an expensive operation ($1430 for Toyota part), one that I'm not convinced is the best move. From what I've read, this is not a cut and dried code. From what I understand, if it started with P0420 and gas smell, the culprit probably would be a bad front side O2 sensor. That rich fuel could lead to a failed cat (combustion in the cat). But, because the original code was P0440, I'm dubious about a sensor or cat problem and am leaning toward it being a loose line or a failed gasket somewhere. I think the next step should be a thorough inspection of the evap system. Unfortunately, I don't have the tools needed for that.
Does this sound like a failed cat? Or does it sound like a leak in the system? Or both? Or neither?
I've driven this Corolla 14 miles every day: 7 on highway, 7 city.
Two days ago, I rescanned the fault codes. The reported fault code had changed from P0420 to P0440. See my other question on this.
I noticed today the fuel smell after the car sat parked for 8 hours. Before, I had only noticed it after driving. Ambient temperature 77 deg F, 55% rel. humidity, no wind.
New engine code today: P0446.
I will drive it some more and confirm these readings, but if my understanding is correct, rear tracking front usually indicates a bad cat. I still need to get under there and take physical measurements (temperature, resonance), but the heat shield looks undamaged.
Used my new laser thermometer to measure the temperature differential after driving for 15 minutes. Average influx temperature measured at 401°F ± 2°F. Average outflux measured at 439°F ± 2°F. Very little difference, suggesting there's little or no exothermic reaction in the chamber.
I also used a rubber mallet for some percussive diagnostic. The chamber rattled when I struck it. I've heard this indicates the filament inside is corroded and disintegrating.
So the open question: what would have caused the failure? I don't see how either P0446 failures (evap vent valve or PCM) or P0440 failures (canister, purge solenoid) would poison the cat.
I know there is a tiny oil leak in the timing mechanism, but it's tiny. I can't see this having any effect.
Maybe I should mentally accept the "catalytic converter is old" answer?