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I've been (increasingly frequently) occasionally getting an O2 high voltage code for Bank 1, sensor 2 on my vehicle - P0138 (2009 toyota matrix, 1.8L). The code would often resolve itself within a drive cycle, so I originally thought it was a bad connection, or dying sensor. As it's increased in frequency, I decided to look more closely, and it does look like even when the sensor is operating correctly, the voltage (and thus lack of O2) is higher than it should be.

I know there is mild damage to the cat - I once limped a few km home on a hard miss on one of the cylinders. A coil died (in hindsight I should have pulled the injector as well) and I was not far from home.

Everything else I can glean from torque appears okay - screenshots included are accelerating (O2 at 0.5v), cruising (O2 at 0.9v - this is the concerning one), and coasting (0.0v).

I don't want to go ahead and replace an expensive cat just because of one symptom, the vehicle is not throwing any other cat codes (no other codes at all)

Is this definitely a cat issue? Could this be as simple as needing an injector cleaning?

https://imgur.com/a/Tn2b0iq

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  • How many miles are on the O2 sensor? Is the voltage signal steady from the O2 or is it jagged? Also, you can check your cat to see how it is functioning. Oct 17, 2021 at 14:41
  • Car has just over 200km on it, I don't know if it's the original O2 sensor, but I've put at least 60k on it Readings jump around occasionally but I'm not sure what O2 readings are supposed to look like. I'll try graphing it to provide more info. Thank you! Oct 18, 2021 at 12:46
  • O2 sensors are usually good to 100k miles (160kkm) at which point they start getting tired. If you've not replaced them, it's something I'd consider doing, especially on the before cat side, as these will cost you fuel mileage in the long run. Don't wait for them to throw a code, replace them at the proper mileage interval as a "general maintenance item". Oct 18, 2021 at 13:40
  • Good to know! 200k probably means they're at their second maintenance interval. Will do the sensors and get a known good reading to confirm whether the cat is working correctly. Thank you! Oct 18, 2021 at 14:09
  • If you haven't done so, read the link I provided, which will give you an understanding away from sensors to know if the CAT is functioning correctly. Oct 18, 2021 at 14:16

2 Answers 2

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O2 sensors are usually good to 100k miles (160kkm) at which point they start getting tired. If you've not replaced them, it's something I'd consider doing, especially on the before cat side, as these will cost you fuel mileage in the long run. Don't wait for them to throw a code, replace them at the proper mileage interval as a "general maintenance item".

If the cat is functioning properly, the after-cat O2 should have a fairly steady reading.

Also, read the answer I gave at the following link. It describes how to tell if the cat is functioning or not while not relying on the sensors to tell you. You can fairly easily get a cheap no-touch (or infrared) thermometer. You're not looking for exact numbers, but rather the difference between the inlet and outlet temperatures.

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The most common cause of this trouble code is failure of the pre-catalyst Air Fuel Ratio sensor. The sensor still works and will show readings and respond to changes in mixture but it has drifted off calibration, is this case to the rich side of the mixture scale. See Toyota T-SB-0001-10. It can be caused by internal failure of the post catalyst sensor but this is less common. It very unlikely that the catalyst itself is causing this code to set. If the code was caused by a rich mixture due to a problem upstream of the pre catalyst sensor, the injectors for example, a code P0172 (rich mixture) is more likely. An exhaust gas analyzer is the best tool test this system but these are less commonly available in repair shops.

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  • This is interesting - so even though the code is showing the downstream sensor as bad, the upstream sensor is actually the one causing issues? Oct 21, 2021 at 15:40
  • Yes, the downstream sensor is just a mixture meter. The code says the downstream sensor is stuck at a high voltage thus indicating a rich mixture. It does not say the sensor is broken. The fact that it will still produced a high voltage in response to a rich mixture is a sign that it is working as designed. These sensors usual failure mode is low voltage as the sensor surface gets contaminated. The post cat sensors fail less often as the catalyst acts a filter for some the particulate type combustions contaminates. Oct 22, 2021 at 7:18

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