What should the O2 sensor voltage read?

Question

hello I have 2008 Nissan Sentra with high mileage and I was looking at the scan tool data for code p0171 while trying to trace the problem and I came across the o2 sensor  voltage readings the o2 sensor before the cat or the one closest to the engine was reading 2.xx volts and the 02 sensor at the catalytic was reading normal .1-.9 mvolts is the voltage for the first o2 sensor normal? I feel like it’s too high idk I need some advice thank you

MountainManJoe · Accepted Answer

Some wideband oxygen sensors (AKA A/F sensor) work differently.

If the air/fuel mixture goes rich, the current increases in one direction (negative). If the air/fuel mixture goes lean, the current increases in the opposite direction (positive). From this, the PCM generates a voltage signal that varies as the air/fuel mixture changes. This signal may range from a low of around 2 volts (very rich) up to nearly 4 volts (very lean). At Lambda, the voltage signal generated by the PCM will be 3.3 volts.

One of the key points to keep in mind about the differences between A/F sensors and conventional O2 sensors is that the voltage signal goes up (not down) when the fuel mixture is lean. Another is that the voltage signal is coming from the PCM, not the sensor itself, so you can’t read an A/F sensor’s output voltage directly with a digital storage oscilloscope (DSO) like you can with a conventional O2 sensor.

https://www.underhoodservice.com/troubleshooting-wideband-o2-sensors/

However, I do not know what you have installed on your vehicle. But I can't understand how a conventional HO2 sensor could generate over 2V.

yaser · Answer

The normal range is 0.1V to 0.9 V.

jack62 · Answer

It should "read" 1.8 volts at 2,000 rpm.

But, it's not an O2 sensor it's an A/F sensor and you're probably using an inexpensive (<$800) scanner looking at OBD2 data instead of OEM data so I wouldn't trust it to interpret the A/F sensor "voltage" accurately (because it isn't even voltage to begin with).

That said, you have a P0171 code and you're wondering if you have a faulty A/F sensor, a faulty MAF sensor, a vacuum leak, or a fuel delivery issue.

Instead or wrestling with that A/F sensor voltage on its own, on your scanner (Live Data) pull up your fuel trims along with the A/F sensor voltage.

Afterall, you just want to know if the sensor is working right and responding to rich and lean conditions, and if it is, is the computer adjusting the fuel trims accordingly.

So the A/F sensor reacts really fast. While looking at your scanner, pull a vacuum line off of the intake manifold to force an even Leaner condition. The A/F voltage should go up and the Short Term Fuel Trim should go even more Positive.

You can test it the other way too. Get one of those hand held propane torches at Home Depot and a short length of hose and attach the hose to that intake vacuum port to force a Rich condition.

Add propane as you watch your scanner. The A/F sensor voltage should go down as the Short Term Fuel Trim goes Negative.

(AF sensor voltages are the opposite of O2 sensor voltages. They go up Lean and down Rich)

Anyway, if you see the A/F sensor responding to the Rich/Lean conditions you forced and the computer is reacting by adjusting the fuel trims, you can rule that out and look for other reasons for the P0171 code.

Stuff like the MAF sensor (use your scanner to see if that looks right)

Speaking of using your scanner, maybe go to Freeze Frame and see what the engine parameters were when that P0171 code was thrown. That could be a clue.

Of course, vacuum leaks including your intake manifold, vacuum hoses, PCV valve and its hose.

Exhaust leaks.

If those check out OK, then explore the fuel delivery side.

Perhaps inadequate fuel pressure or an injector that's clogged.

People don't like to do it anymore because with COP coils it's a pain in the butt, however, examining your spark plugs can give you a huge clue as to if this lean condition is being caused by just 1 cylinder