TL;DR: O2 sensor data, thermal measurements, and physical resonance tests all point to a failed Catalytic converter.

Here's the back story. MIL illuminated with codes P0440, P0420, P0404. Around this time, I noticed a gentle mechanical rattle/plinking on acceleration.

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.

Symptoms now:

  • 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?

Update 3/14

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.

Update 3/15

New engine code today: P0446.

Short and long term fuel trim are erratic this morning during city drives: fuel trim

The over-time O2 graphs shows rear sensor (green) tracking front (red): O2 sensors 1 and 2

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.

Update 3/16

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?

  • Just thinking about the symptoms, this is a 2003 so I'm assuming it's throttle body or port fuel injected. Where would the smell of fuel come from? Is it being burped back out the air intake as the engine stops, or is it coming from a leaking intake path? Maybe a bad gasket, or cracked intake manifold? Could it be coming out the tailpipe? I'm grasping at straws here trying to figure what would make sense to look at next.
    – cdunn
    Mar 10, 2016 at 16:25
  • 2
    How can I tell if I have bad cats?
    – Zaid
    Mar 10, 2016 at 18:50
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    Just a warning: I once went from "performance seems a bit down, fuel consumption seems a bit high" to "engine will start easily, but stalls on any attempt to increase revs beyond idle speed" within the space of about 50 miles while driving. If the problem is a disintegrating cat which is blocking the exhaust with debris, it can get worse very quickly and without any more warning signs. A successful get-me-home fix was to unbolt the exhaust from the manifold and insert a few washers, between the two parts, thus effectively having no exhaust system at all. Noisy(!!!), but at least driveable.
    – alephzero
    Mar 10, 2016 at 20:11
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    @IhavenoideawhatI'mdoing Pulled that graph. Downstream tracks upstream. OP updated with further data.
    – bishop
    Mar 15, 2016 at 13:45
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    Does look like non-functioning cat. But at $1400 I'd quadruple check. I've found some of the tips on this page useful, might help you as well: discountconverter.com/tech-tips/… Mar 15, 2016 at 14:53

3 Answers 3


If you have rattling on acceleration and the fuel economy is depressed, it is very likely that the catalytic converter core has cracked and it is blocking the exhaust now. You can try to measure its efficiency first by looking if the rear O2 sensor voltage reading after heating is steady (so the catalytic converter is good) or jumping (so it is bad). If it is too much trouble for you to do this test and you can remove the catalytic converter yourself just do it and inspect it to see if it is broken. Mind that some debris may have been flung into the muffler so it may also need to be cleaned or even replaced.


It definitely sounds like you have a serious issue with the catalytic converter and should have it replaced. Replacement with a universal converter should not be expensive. In expensive Silicon Valley (and air conscious CA) I got a replacement for $220 at a specialist shop.

However, the failure of your catalyst is a symptom of another issue and not a cause, as something clearly caused the catalyst to overheat, crack and fail. Once your catalyst is replaced, you should waste no time in determining if there are any other codes that persist (or newly appear) that indicate a root cause.

In my case, I had a P0171 (System Too Lean) that basically meant that the ECU detected a condition that causes the engine to misfire frequently, which heated up the catalyst beyond specification and caused it to fail. The root cause was an oil-covered Mass Air Flow filter which detected less air than was actually passing through the air intake. The oil came from a "performance" reusable air filter. Never again.

Also, the 1ZZ-FE engine is notorious for burning engine oil because of pistons that are too small for the bore but Toyota cheaped out and compensated by using larger piston rings, which wear down and/or stick. The burnt engine oil further poisons the catalyst leading to a P0420.

  • 1
    You know, I never even considered using an aftermarket part. The Toyota part is $1430. I am not joking: fourteen hundred bucks. There are many CARB compliant after market cats, so I could choose any of those.
    – bishop
    Mar 11, 2016 at 0:51
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    I'm having the same nightmare with my Prius now, except that there is no legal aftermarket catalytic converter. The only thing that I can do is to cheat the test. Mar 11, 2016 at 1:37
  • @gabrieldiego Yikes, I own a Lexus CTh (same drivetrain). Hopefully someone gets around to making an aftermarket one that's CARB compliant. Mar 14, 2016 at 22:33
  • Well, cats do fail because of age. Mar 15, 2016 at 9:07

Essentially the technical meaning of a p0420 is that the downstream o2 sensor is detecting an inconsistency with fuel trim. Generally this is code is associated with a bad catalytic converter. Many things can cause this code, but usually the easiest ways to figure it out are testing it with a hydrocarbon analyzer. The dealership usually has one of these. If they don't, all you can really do is test exhaust back pressure which is the pressure created inside the exhaust before the Cat. If you detect that it's too high, this would indicate a blocked or damaged cat.

Cats have honeycomb filters inside of them, when they break they turn sideways or will even get stuck in the outflow pipe of the Cat.

I'm not too savvy with emissions data. However though, I met this guy at SEMA and he knows a lot about emissions systems.


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