I'm trying to get a measurement of the voltage on my 1995 5.0 EFI. It's a California truck but has only one O2 sensor at the Y pipe. I have the Breakout Box on the truck and get meaningless voltage readings at pin #29 or #44. The O2 sensor is the four wire variety.

 

If your asking my truck which isn't a California emissions one has a single O2 sensor on the exhaust there normally is just one. If you want to read voltage from the O2 follow the wire to where it connects or probe it at the exhaust. And your breakout box could be incorrectly wired or setup.

 

I would expect to see 0.45 volts on my DVM on pin #29 or, in the case of some other trucks of this era, #44. Back probing all four at the O2 sensor I 14v, 0v, 3.45 v and 0.07v.

I guess what I'm asking is can you even get a reading of 0.45v from this multi wire setup. Of course two wires are the heater. My 1986 one wire 02 sensor gives the 0.45v on one of my DVMs. My higher frequency unit will spit out voltages between 0.1 and 0.9. averaging 0.45v.

I've never probed a 02 sensor with more than one wire before. Are the findings I'm looking for even obtainable on a 1995 truck

FWIW: breakout box is correct. Hard to screw up if it's even possible

 

The voltage switches above and below 1/2 volt on a properly operating system in closed loop
If you ever have one stuck at say .27v, that would be considered "slammed lean"
If you have one stuck at .87v that would be considered "slammed rich"
There is some info in older books, and here is some of it from a training manual from the eighties
You have a breakout box and that is a real good start
The sensors switch as fast as you can see digital numbers on a screen
Back and forth above and below 1/2 volt (up to 1 volt) (less than 1 volt, .99v)
They figure if .5v is stochiometric ratio, (14.7:1) switching above and below will result in an average of .5v
The other 2 wires on that O2 sensor is the heater circuit and that makes the sensor a HEGO
The faster they heat up, and start generating voltage,the sooner the processor can go into closed loop operation

 

 

Original sensors had 1 wire [actually shielded, so two]. They were installed close to the exhaust port so they they could heat up and turn on. They usually had a 30k service interval because they 'burned up' because they were too close to the exhaust port.

Someone thought it would be a good idea to move the sensor further down the exhaust so it does not get burned up and put a heater in it to get it up to temp to turn on. So, you went from 1 wire to 3 [heater works on 12v and ground].

Then they found out that O2 sensors did not always worked right because they found that although the exhaust system appears to be grounded to the vehicle, sometimes it was not. They installed an extra ground wire to assure the sensor was grounded. That is the purpose of all 4 wires.

O2 sensors tend to short out when they get old, not all the time, but enough that any seasoned tech knows about it. When the signal wire is shorted, the ECU thinks the engine is running lean. To get the fuel mixture right again, the ECU keeps pumping more and more gas. The ECU has no clue the sensor is shorted. This is why many engines run really rich and once the O2 sensor is unplugged, runs fairly well again.

The new wide band sensors are at least 6 wires.

 

I think this is your pinout:

 

Thanks for confirming this. Something I had only assumed.

 

It's a 1995 California 5.0 MAF setup.

Truck is now running. I had decided that the 0.007 volt reading was equivalent to no voltage. The determination was an O2 sensor that was putting out no voltage. I had a new O2 sensor on the shelf so I installed it.

 

When the engineer asked me to check the exhaust for ground, I thought he was an idiot. WTF! Can't you see it is? Well, it was not, it has well over 1k ohms resistance. I guess we know who was the idiot. Back in the day, we put on a ground wire at the O2 sensor. Wire attached with a hose clamp. It sure looked butchered, but it did fix the O2 sensor code that we could not resolve. A couple years later, they had 4 wire sensors.

I was pretty young back then and those years, I learned a lot about testing vs assuming. Testing paid more and had less comebacks.

 

It's not possible to measure an O2 sensors output with a regular multimeter it requires one with an oscilloscope function, the output of the sensor switches fairly rapidly between 0v and 1v and a regular voltmeter is just not designed to read that kind of signal.

 

I would agree, but most people don't have a scope and many DVOM's have a min/max scale which can be almost as good. Although I do have a scope, typically, when an O2 sensor fails, it's not going to cycle or it will cycle very slowly. A standard DVOM certainly may not tell you your O2 sensor is good, but it can tell you if it is bad.

O2 sensors don't last forever. As far as I am concerned, they are a wear item. If you don't know when the sensor was replaced, have a code that says the sensor is faulty, or when the sensor is unplugged, the engine runs better, I don't think you are wasting money buying one. Unless you are buying one that says Duralast.

Yes, you can't test an O2 sensor properly without a scope.

 

A dvom is sufficient to backprobe a O2 sensor and come up with the voltage switching readings
An oscilloscope is great for graphing but not needed to read O2 sensor voltage

 

I mentioned in post #3 above that both my DVMs measure the O2 voltage on my one wire 1986 EFI truck.
I didn't attempt to check the voltage of the new sensor on this 1995 truck.

 

So is it switching like it should be?