I have a 2002 Toyota Corolla (engine 1zz-de) and I am trying to get to the bottom of a p0171 code. I've read from other posts that a faulty O2 sensor could cause the lean condition, so I have been testing those sensors with my scan tool. I was hoping someone could take a look at the screenshot attached and tell me if it looks like a faulty O2 sensor

  • Test is made at idle after engine warmed up
  • I am simply recording the O2 sensor (bank 1 Sensor 2) voltage
  • I am seeing it remaining pretty flat for the first 20 seconds or so, then oscillating a little bit but not evenly between min (0.1) and max (0.9)
  • Based on what I've read, this shows the sensor is faulty
  • Question #1: do you agree that this sensor is faulty?

Note: my other O2 sensor (bank 1 Sensor 1) is oscillating much more nicely, with a 2.5 sec period. - Question #2: should the sensor not be oscillating much faster, with a 1 or even 0.5 sec period?

enter image description hereenter image description here

  • can i ask you wich app you used for that scan?
    – Rolando
    Nov 11, 2018 at 13:37

1 Answer 1


These sensors look normal. Bank 1 sensor 1 should cycle about twice per second, this is a bit slow but fast enough to maintain normal fuel control. It is common for Toyota to cycle the system a bit slower at idle. It would likely cycle faster at a higher rpm. Perfect symmetric curves are not normal when viewed on a scan tool, the data sampling rate is not high enough. They would look more symmetric on an oscilloscope.

Bank 1 sensor 2 is expected to remain a flat line with only small wiggles if the catalyst is lit and working properly. This line can be at any voltage in its operating range from rich to lean. This voltage can and will change over time and load as the PCM requires for power or emissions. We see here a normal sensor but a catalyst that is not fully warmed up or is beginning to fail.

Cool data graphs, Consider running them again at 2500 rpm. For even better data run the test at steady cruise speed while driving. But please do not "Scan and drive" get a helper.


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