P0420 Diagnosis, Bad O2 Sensors, Bad Cat, or Other?

SwarlosSwarlos Member Posts: 2
edited March 2021 in Toyota
I have a 2004 Toyota Corolla that's throwing the P0420 error code, Catalyst System Efficiency Below Threshold Bank 1. The engine has 300k miles on it, and has been very good to me for a long time. I'm new to auto repair, but I'd like to get the proper diagnosis on this before throwing a lot of money at it.

I put in new spark plugs (NGK 4996) after seeing this code pop up because I did a little bit of research, and that was the cheapest/easiest thing to fix. Plus I couldn't remember how old the spark plugs were. I can try to link a picture of the old spark plugs if anyone thinks it's relevant. They looked worn out to me, but not ashy/oily.

I bought a cheap OBDII scanner to try to figure out exactly what's going on via O2 sensor voltages. For reference I also have a 2006 Corolla with 150k miles to compare O2 sensor readings.

On the 2004 Corolla the upstream sensor reads (after warm up ECT>167F) from 55mv to 780mv at idle with new readings every second or so. On the 2006 the upstream sensor reads from 3.25v to 3.35v at idle with 4-5 readings per second.

The '04 downstream sensor reads from 0mv to 820mv fluctuating about every second with the numbers biased toward the higher end. I have a video of the readings on the '04 sensors if anyone should care to see it. The '06 downstream sensor reads very steads from 500mv to 800mv, the numbers update much less often and slowly creep up while idling rather than bouncing up and down.

The 02 sensors all check out as OK based on the monitor tests on both Corollas. Where to go from here? Any help would be greatly appreciated.


  • SwarlosSwarlos Member Posts: 2
    A little more information, The '04 Corolla on a cold start (about 35 degrees) will fast idle up to 2000 rpm which slowly drops down to about 700 rpm after warming up to 167 ECT. The '06 Corolla will fast idle up to 1600 rpm and drop down to 700 rpm at a similar rate.
  • thecardoc3thecardoc3 Member Posts: 5,614
    edited March 2021
    New readings every second or so is too slow of a data update rate to really be useful in most cases. In order to decide if the catalyst has failed the computer watches the downstream sensor in order to determine how long it takes a slightly rich command and then slightly lean command to deplete stored oxygen and then replenish that oxygen level in the catalyst. The ability to judge the health of a catalyst when scan data updates in real time with professional level scan tools is arguable, one that updates much slower than that would be very misleading.

    The 2006 is using an Air/Fuel sensor and not an oxygen sensor. The system tries to keep the A/F ratio at stoichiometric which is reported by a scan data value of 3.3v. So what you wrote above is normal but with experience one learns that if that sensor is skewed can result in fuel control errors. The 2004 does use a conventional O2 sensor and it should be switching about 3-6 times a second. Your scan tool is only updating fast enough to show you a fraction of that.

    There is way more to this that just this brief explanation but with a P0420 we first do have to prove if the fuel system is working correctly. We have to confirm the accuracy of the O2 and A/F sensors. Make sure that there are no exhaust leaks or software updates for the engine controller and if all of that is OK then the only thing left to do is trust that the onboard system has tested the catalyst correctly.
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