I'm getting code P0420 on an '03 Vibe. The rule of thumb is that this means the catalytic converter is gone. Yet the pre- and post- cat O2 signals appear to indicate that the cat is doing its job.

O2 signals

Unfortunately Toyota/GM have designed the entire exhaust manifold as a single piece, a part whose cost exceeds $1000. I'm not sure that pulling an old cat from a graveyard and splicing it in would be worth it. It may not last and the labor cost would be high.

But given the O2 signals shown above, it's not even clear that changing the cat will get rid of the CEL and the code.

What else could be malfunctioning and causing the P0420 code?


To assist SteveMattews (and other helpful folks) I'm adding a picture showing what the post cat O2 sensor and cable look like. The 90-degree angle that the cable makes at the point it enters the chassis indeed looks suspicious.

post-cat O2 sensor and cable

  • Perhaps an obvious question but after you've cleared the CEL and fault codes with your scan tool, how quickly do they return? Aug 16, 2016 at 14:11
  • @SteveMatthews Good question. I expected maybe 30-40 miles until the code will return. It took all of 125 miles of driving on the highway (at about 70 mph) for the code to return. Does this provide a useful hint?
    – Calaf
    Aug 16, 2016 at 14:15
  • 2
    Check the condition of the wiring loom to the oxygen sensors. I had similar behavior and traced it to a frayed wire which was sometimes disconnecting when the car was moving. Aug 16, 2016 at 14:39
  • @SteveMatthews Very logical suggestion. With home electronics I am used that the most likely cause for no contact is dust within a connector. There it is usually enough to pull/push the connector 5-10 times. The little dust particles present (preventing proper contact) abrade the copper. When some fresh metal is exposed, the connection is robust again, and the dust particles fall off in the process. Wires in something like headphones can all but not be repaired. They are made of supremely thin copper around polyester.
    – Calaf
    Aug 16, 2016 at 15:01
  • @SteveMatthews Could you clarify whether the connectors could be the culprit, and whether the copper wires leading to the O2 sensors are repairable?
    – Calaf
    Aug 16, 2016 at 15:01

1 Answer 1


I don't think the secondary O2 sensor is reading properly. If the cat were actually shot (a legitimate P0420 code) the bottom graph of secondary O2 should be very similar to the top "primary" O2 aka "Sensor 1".

Keep in mind the cat needs to be fully heated for these graphs to be the most useful. Your pictures look like the Torque cellphone app for a bluetooth OBD2 reader, so I'm assuming you can capture during highway cruise.

In any case, a properly working cat (and downstream O2 sensor) should yield a reduced-amplitude output quite similar to the upstream output. It should rise and fall, and transition roughly the same time.

Your secondary looks pegged rich. Unless the cat was cold, I suspect the O2 is bad. I would not suggest trying to repair the wiring, even if you discover a specific problem.

  • The post-cats should read rich. I'd say they're fine here.
    – Zaid
    Aug 16, 2016 at 16:59
  • I agree with @Zaid. The post cat when cat and O2 are working correctly should be fairly steady. Here is a graphic of what I'm talking about. Aug 16, 2016 at 20:57
  • @Pᴀᴜʟsᴛᴇʀ2 I don't like that cheesy drawing at all. A straight line means the O2 sensor is broken This shows what I meant: that the downstream and upstream are in sync, with the secondary waveform having most of the amplitude damped, but still responding in sync with the upstream sensor. This is an actual waveform output, not a line drawing. ronstultz.com/knowledge/images/…
    – SteveRacer
    Aug 16, 2016 at 22:17
  • That's fine, but even yours is relatively flat. Even the OP's post cat has more fluctuation than what you're showing. My point is, what you originally said was it should follow what the upstream is doing, which I cannot agree with. In yours, it dips somewhat ... I'd call that a waggle, but not following. Maybe it's just interpretation on both our parts though. Aug 16, 2016 at 23:39

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .