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O2 reads maximum voltage with KOEO?

Very strange thing happening here. When I shut my engine off, both O2 output voltages on a Scan Tool drop to zero, which is good, and normal? But, after sitting for 8 hours or more, if I check again before starting the car, somehow they both show 1.25-1.27 volts!? How is this possible? It’s a 98 Civic EX with regular 4-wire heated sensors front and rear.


Are you putting the key to “ON” to power them when you read them before starting your car?

Umm, yes. But O2 sensors are not “powered up”, the are simply supplied a ground and create their own voltage output as a result of oxygen content, or lack there of. The heater is supplied 12 volts and ground as well, but thats it.

Like I said, when I shut the car off but leave the data display on, the front O2 goes to zero right away, the rear takes up to a minute but eventually ends up at zero as well. I just dont understand how after that, the voltage slowly goes up, millivolts at a time, until after a few hours they are back up to max output of 1.27 volts?

Good question. Remember, the sensor only produces its own voltage when properly heated (~600°). What you may be seeing is as the sensor cools it stops working and you’re seeing the effects of current leakage from your DVM biasing the elements.

It is weird but I’ve never thought to look at or think about a KOEO O2 sensor voltage.

I’m sure mountainbike knows that an O2 sensor generates its own voltage. But that’s only true for the signal voltage. They are also heated, of course, so voltage is being sent to the heating element and he may have had a thought about that.

How long is the key on before you get this reading? What does it do if you just sit there with the key on?

Here’s a theory - half baked though it may be. Those heater elements can work pretty fast. You turn the key on. By the time you actually get a reading you are getting an actual voltage readings which are very high because the last of the exhaust trapped in the pipe is basically “super” rich. Its probably totally nuts. (Can an O2 sensor actually produce a voltage greater than 1V?) But that’s what I’ve got.

There’s probably some reason you’re scrutinizing the sensor readings. Maybe if you told the whole story it would give people more food for thought.

Cig, that’s exactly what I was thinking.

And I agree with you that we could be a lot more help with more info.

TSM, we should just open an auto repair shop together. We could agree on most of the auto repair/maintenance issues while we disagree about most of the politics & economics of it all. I think it would be a raucous and entertaining affair. We’d get customers just for the entertainment value!

Thanks for the chuckle Cig. I have a feeling you’re right!

OK nevermind, it’s apparently normal. I did the same thing on a 2002 Civic Si and it was also 1.270 volts on both front and rear O2’s with the KOEO…so all is normal…

Like said above, the O2 sensor has to reach it’s operating temperature before it’s output has meaning. This is probably normal behaviour.

But George, before it reaches operating temp it shouldn’t be producing any voltage at all - it can’t.

I’m not telling you that I know what the explanation is. But before reaching temp it just can’t have any output - “meaningful” or not.

The reason you’re seeing the higher voltage output from the O2 sensors with the engine cold is because the computer is in the open loop mode. So it’s not even measuring the voltage output of the O2 sensors. Not until the engine reaches operating temperature and the computer goes into the closed loop mode where it starts reading the voltage output of the O2 sensors will you get an accurate reading.


I think TwinTurbo has the correct answer. What you are seeing is voltage being leaked back by the ECU circuitry. If you want to prove it then disconnect the O2 sensor and see if the voltage still appears. I think it will.

Cougar is correct, in the diagnostic test procedure for the oxygen sensor it states the voltage should read greater than 1.00 volt with the oxygen sensor connector disconnected. This likely for diagnostic purpose to monitor the circuit for short to ground before the engine is started.

Thanks for explanation Nevada_545. I didn’t think about the diagnostic part of it but what you say makes sense.

Being searching the internet to try and figure my issue out. And this is the first place that has someone measuring 1.25 from scan data.
I see this is a old thread but crossing fingers

1998 Honda Civic 1.6

Primary and secondary o2 sensors newer.
Recently the car has been bogging at light cruise 40-50mph
Then it suddenly goes away and has power when the throttle was same position the whole time. I have not data logged what exactly what the ecu is doing.
I’m assuming it’s trying to run a test on secondary 02 sensor.
Then some times it will set a p0136 and 138 right after the bogging disappears.

I’m confused because the data for sensors does read 1.25 both front and rear on start up.
Then primary starts to switch as it goes into closed loop. Which shows on scan tool data
But the rear continues to read 1.25 and I have even unplugged it and it’s .5-.6 range directly from sensor.
But plugged in still reads 1.25 scan data

I haven’t gone as far to read the sensor signal at the ecu. Possibly a break in the wire or internal ecu damage( water etc)

My parents have a similar car and it as well starts with 1.25 for both sensors. But it’s secondary O2 heater circuit is open and setting code.
So I can’t fully test it to see what happens to the readings when

Thanks for any tips.

What type of O2 sensor technology does your Civic use front vs rear of the cat? If they are the same, first off, make sure the heater circuits are powered and working correctly, and that the sensor grounds are making good contact.

Try that same unplugging experiment with the front sensor.

Thanks George

The 98 civic uses a 4 wire heater o2 sensor primary and secondary.

I will have to see what the voltage is unplugged. But if I had not mentioned testing on similar car the primary and secondary at koeo cold. Was 1.25 data stream readings.

As for ground I will have to verify if the ground are completed at the ecu from the o2. The heater circuits are grounded at the ecu.
But as I noted before even with the o2 sensor disconnected I was reading voltage from the secondary. I will have to retest but possibly I did a tap into the signal line. And was reading .6-.5 voltage. And the data stream said 1.25 from the ecu.