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Hello, A few weeks ago my Toyota Sienna (V6, 3.5L engine) threw a P0158 error – Bank2 Sensor 2 (downstream O2 sensor) stuck in high voltage.
I replaced the sensor but the error came back.Watching voltages via OBDII reader, bank 1/s2 and bank2/s2 appear to behave
more/less similar with the bank 2 sensor showing the following differences:Seems to be more “sticky” but only at rich values. When I rev the engine to
4000RPM and release the accel. it drops to 0 just like the bank1s2. However,
then it recovers a lot quicker than b1s2 and tends to go higher than b1s2.At the same time, B1S1 and B2S1 are very close in variability iwth B2S2
having a little higher values. E.g. 3.27 vs 3.37 at 4000rpm, both are at 5.00V.
then, after the RPM drop they go down to 3.12/3.25 respectively, etc.Can anyone give some tips on how to further diagnose this? Visually catalytic
converter seems fine on Bank2.Trims, by looking at live data, show :
Short term trim swings from -20% to +5% which is very similar on both banks. Long term trim on bank 2 is different than B1. On b2 I get from +4.7% to +7% In contrast, on bank 1 it’s between -2.7% and +4.7%Thanks anyone who can give me some pointers where to look next.
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Yes, replaced with Denso part 234-4209
Right now I’m at a point where I suspect the sensor is defective. I bought it new but maybe it was defective from factory.
I would think a bad new Denso O2 sensor would be a long shot.
I would recommend a thorough visual inspection at the O2 sensor plug and go as far back as you can.
If the mixture is truly that rich, there may be an issue with a fuel injector or a cylinder not firing.
Sorry, but this one may not be real easy.
Good luck
-Karl
Yeah. Took off the B2S2, inspected it again, using ohm meter the heater looks good, the sensing side looks good. Voltage on the sensor connector looks good: heater voltage +12 with key in, engine off. Heater ground not connected to vehicle ground (to rule out heater being constantly powered).
Verified all 3 cylinders (2,4,6) in Bank2 actually fire (by disconnecting ignition briefly and observing RPM disturbance), also by briefly disconnecting fuel injectors. In all 3 cases B2S2 detected very lean very shortly after a fuel injector was off-lined which suggests to me that O2 sensor is doing it’s job.
Verified all 3 spark plugs on bank2 – look good and actually throw spark when tested each with their ignition coil.
Which leads me to believe that I do have a rich condition.
Now, for a rich condition I would expect B2S1 to show odd behaviour, different from bank 1, but it shows very similar behaviour as B1S1, observed through OBDII. This is not the same as testing it with an oscilloscope which I don’t have so I can’t be sure it’s good.
I ruled out any air leaks since that, I would expect, would affect both banks more/less symmetrically.
Which leads me to believe it’s one of:
a) A/F sensor on bank2 is wrong and biased towards lean reading (i.e. it is reading more oxygen than there really is). It seems to work, but gives wrong (biased) readings.
b) A fuel injector is not spraying causing occasional combustion failure, letting raw fuel reach the oxygen sensor. But then, I would expect, A/F sensor would react and ECU would command lean.
So, I’m inclined to believe it’s a) – A/F sensor failure. Kind of expensive guess to just go ahead and replace it: the sensor is $200 CAD and no returns on electrical parts.
Thoughts?
Forgot to mention: the car runs and idles just fine. No perceivable loss of power, lag, rough idle or anything. Runs great.
Exploring further the possibility a) “faulty a/f sensor”, I logged A/F sensors on both banks. As can be seen in the graph, Bank 2 is offset and slightly above 3.3V.
My understanding is that above 3.3V it means “lean” and below 3.3 “rich”. So, if that is the case, it could explain bank 2 being commanded more fuel (to compensate for ‘lean’ reading).
But since A/F sensor is not stuck, only biased, all seems to work fine except for the downstream O2 sensor on Bank2 which, in this case, rightfully shows “rich”.
Does this make sense?
Thank you for your reply.
Yes, I had inspected exhaust – all was good. I inspected *many* other things to no avail. And then, finally, I inspected the obvious (and easiest).
Now, in retrospect, it is obvious and easy but took me a while. I would have saved a lot of time and some $$$ had I done the obvious tests first. So here is my full update for those who might stumble upon this thread:
P0158 reports downstream sensor high voltage. My advice: DO NOT REPLACE this sensor just yet.
First, test the easy things:
a) Disconnect O2 sensor and check its heater resistance. (two black wires) – approx 10-15 ohm or whatever you find online it should be (it should be something like that, so approx 1A current flows through it)
b) Disconnect A/F sensor and check its heater resistance (two black wires) – approx 3-5 ohm (changes with temperature, up to 8A can flow through it).In my case, it was the Bank2 Sensor 1 – Air Fuel sensor heater that was open. Looking at the diagram I posted above it is now kind of obvious that it is *that* sensor that is giving faulty readings – it is a lot more jittery compared to Bank1. Since it operated with no internal heater, the readings were swinging with the temperature which changes a lot depending on the load.
So, my downstream sensor that I replaced was almost certainly good – even though the error indicated its failure.
Thanks to those who replied.
It definitely did not. Not sure if it is detecting open AF heater circuit at all – my guess is it does not and the a/f sensor being so close to the cylinders does get very hot fast so the readings kind of looked correct.
In any case, no errors since I replaced the upstream sensor, all looks good now.
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