Sanity checking if I need a new O2 sensor for P0138 (o2 sensor high voltage)

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Frogblast, Nov 7, 2021.

  1. Frogblast

    Frogblast Junior Member

    Nov 22, 2019
    2006 Prius
    My prius is now at 250K miles. About a month ago I started getting P0420, and sometime after that a P0138 appears. It wasn't immediately clear if the problem was with the cat (which has been replaced 100K+ miles ago), or with the o2 sensor.

    Upon resetting the codes, the P0138 reliably reappears within minutes, long before the Prius can redo the catalyst test. That definitely makes me want to try replacing the O2 sensor before doing anything with the cat (esp. since in CA I can't get a cheaper non-OEM copy). But I gathered some sensor data while driving, and it makes me doubt that conclusion a bit.

    1. Driving up a steady incline. The spikes are where I floored the gas for 2-3 seconds:

    2. In Diagnostic Mode, running up and down between 1500 and 2500 RPM:

    3. Driving immediately after resetting the MIL, this was taken right after the MIL re-lit. I think this time window probably captured when the test was done:

    The freeze-frame data from when this trouble code appeared is:
    $02 - Exhaust Gas Sensor Monitor Bank 1 – Sensor 2 TID $07 - Minimum sensor voltage for test cycle (calculated)
    measured: 0.895 V
    min: 0 V
    max: 0.233 V FAIL

    $02 - Exhaust Gas Sensor Monitor Bank 1 – Sensor 2 TID $08 - Maximum sensor voltage for test cycle (calculated)
    measured: 0.895 V
    min: 0.605 V
    max: 65.535 V FAIL

    What conclusions can be drawn from this?
    1. To raise P0138, the car alternates slightly between lean and rich, and the test wants to see corresponding voltage fluctuations in the downstream O2 sensor. But note that both the min AND max voltage observed is 0.895. It didn't move at all. And my graph of O2 sensor outputs (image 2) shows the downstream o2 sensor voltage flat too.
    2. But, during normal driving conditions (image 1 and image 3), I do certainly observe the O2 sensor voltage jumping between 0.1 and 0.95v as I modulate the gas. So the O2 sensor is indeed responding in a reasonable way.

    Is this the infamous "lazy O2 sensor"? Perhaps it just isn't responding to the small perturbations that the test cycle uses, or responding too slowly, but still responds generally to big swings between lean/rich as I get on and off the gas? What other theories might suggest doing anything other than replacing the O2 sensor?

    And related question: I'm pretty sure both the A/F Sensor and the O2 sensor are original equipment at 250K miles. Should I go ahead and replace the AF sensor too while I'm at it? Note that in both Images 1 and 2 it spikes above 5V under very lean conditions, when the tech manual suggests it should live its whole life between 3.0 and 3.4 volts.

    (hopefully this is indeed the problem, and that'll fix the P0420 too..)

    One final complication: About 5 minutes before the P0420 first appeared, I did have something strike the underside of the vehicle. I got under there and saw no dents/scratches/non-surface-rust/etc, anywhere on the exhaust. I hate to write this off as a coincidence, but I'm not sure what else to look for. Is there any other way to try to detect leaks other than visually?
  2. Tombukt2

    Tombukt2 Active Member

    Nov 29, 2020
    Durham NC
    2009 Prius
    Maybe leaking at the spring connection at manifold to down pipe

    SM-A715F ?
  3. mr_guy_mann

    mr_guy_mann Senior Member

    Sep 3, 2020
    2006 Prius
    So, short reply is that I would suggest replacing both the AF and O2 sensors with OE or Denso aftermarket parts from a reputable source (not ebay or amazon). Then see what happens. If one or both sensors are off, then the cat monitor results will be off. (That don't mean that the cat is ok, but you can't tell-yet).

    Longer reply. The O2 sensor can produce a voltage based on the oxygen content (air-fuel mixture) of the exhaust gases that it "sees". At just over 14.6:1 (lean) it is at 0V, while just under 14.8:1 it produces almost 1V.

    The Air-Fuel sensor works by the ECM maintaining a 300mV differential across the sensor, then measuring how much current is required to do so. The actual mixture of the exhaust gases affect the amount (and polarity) of that current. On just either side of 14.7:1 it is 0 +/- 100 micro-amps. At fuel cut during decel (full lean) it will be 2-3 milli-amps. The software in the ECM "converts" this tiny current measurement into a voltage reading to display in the data list.

    At a steady throttle condition (such as idle or cruise) you should see around 3.3V +/- 0.2V (brief spikes up or down are normal) Heavy throttle (rich condition) should make it go lower - under 3V, while full lean (decel fuel cut) should show 5V.

    Now the downstream O2 will typically read 0.7-.8V when the cat is working normally and is "lit off" by having enough O2, HC, and heat to do its job and convert those gases to CO2 and H2O. If the O2 sensor CAN go full range but is reading higher than it should then I would suspect that the AF sensor is "biased" and is no longer accurate. So it fools the ECM by reporting slightly lean- the ECM responds by (incorrectly) going rich in response-to bring the AF sensor back to where it wants. The downstream O2 (correctly) reports the (now too rich) mixture but the ECM is programmed to believe the AF sensor first, not the O2, so it logs the P0138. This is not an uncommon fault for some Toyotas.

    I usually check for exhaust leaks by raising and safely supporting the car and listening underneath (with an "open" mechanic's stethoscope if needed) around suspect areas while the ICE runs. It can help to have a second person briefly cap the tailpipe with a folded towel or a leather welding glove.

    Posted via the PriusChat mobile app.
    SFO likes this.