school me on testing 4-wire o2 sensors in a GenIII

[YJ_OVER_SJ]

I've got a 2000 Sierra 4.8L (red/blue computer) with a aftermarket harness and tuned computer.

o2 sensors are 4-wire square plug connectors. harness wire colors are pink(reading 12v heater), black(grnd), tan(reading ~4-5 volt ho2s low), purple(reading ~4-5 volt ho2s high).

I took the vehicle to a shop a while back and he told me my o2 sensors weren't working (only holding steady .5v which would align with standard 450mV). now that I think about it perhaps the vehicle wasn't warm enough to be sure. So I did a ton of reading to find out how to test o2 sensors. They do a great job of teaching how to test 1-wire, 2 & 3 wire. But something just isn't right about how that translates to a 4-wire setup.

Like I said, they have ~5 volts running across what I understand is the high o2 and low o2 wires. as the vehicle heats up, that voltage changes in both wires. If I remember correctly, it was slowly increasing as the sensor heated and then dropped to bout a steady 4.34v or so. once it hits the steady voltage, changing rich/lean(pulling vacuum, unplugging coil pack) doesn't seem to change the sensor voltage.

How do I test this o2 sensor to see if it's working correctly? looks like a diagnostic computer could read it off the PCM but how can I do it manually at the terminal?

Ahh, and according to the alldatadiy electrical diagram the tan wire shows as being grounded in the PCM, the purple wire connected to tan wire via variable battery voltage inside the o2 sensor so I'm guessing I do not need to worry about grounding @ the body of the o2 sensor?


btw, here's the test suggested in alldatadiy:
The Heated Oxygen Sensors (HO2S) are mounted in the exhaust system where they can monitor the oxygen content of the exhaust gas stream. The oxygen present in the exhaust gas reacts with the sensor to produce a voltage output. This voltage should constantly fluctuate from approximately 100 mV , high oxygen content = lean mixture, to 900 mV , low oxygen content = rich mixture. The HO2S voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates the fuel pulse width command to give to the injectors, lean mixture/low HO2S voltage = rich command, rich mixture/high HO2S voltage lean command.

[wheelingyota]

first need a lab scope, then warm vehicle up, back pin the signal wire (try both). i have found the best setting for the lab scope is .1 volt per division; 500ms per division usually raise the RPM to around 2500-3000 grand. watch the lab scope screen it should toggle going from lean to rich or rich to lean. should never drop below 0 and the middle should be around 350mv any higher it is biest lean any lowered then 350mv it is biest rich. your also going to look at the O2 rise time thats from where you seen the scope toggling to where you snap the throttle back to idle the scope will basicly flat line then go back to toggle. your checking from right where it raises to where it stops to drop back down. the rise time i have seen any where from 100ms (basicly new) to around 300-350 (getting pretty slow but still working). with out a lab scope your shit out of luck.

EDIT: if we are dealing with wide band (usually new vehicles some 2009 and up or after market) then there is really no good test that i have found.
here is a good picture off google to show what your looking for as of just the pattern

here is a rise time. the spotted lines are around the rise time

[Bo185]

I think the GM tech II reader will read it real time.

Are you getting a SES light for the 02's??? If they are original high milage units I would replace them anyway they do were out.

The Front O2's sensor swings from about .250V to .900V, adjusting from rich to lean detection. The post-cat sensor should have barely any voltage swing at all, if it does, it means the catalytic converter is defective and not cleaning up the exhaust. Which sensors are supected bad the front or rears of cats.

Any deficiency in any sensor will set a SES light.

[YJ_OVER_SJ]

I only have the 2 front sensors on this setup. it's in a buggy so no lights. my odbII terminal is currently on the fritz so can't run a GM tech II.

Was hoping to just read voltage like stated with the .250v - .900v variation but if I'm guessing the complexity of the new 4-wire o2 sensors correctly, I can't simply tap in to one of the 4 wires and test for voltage fluctuations. Guess I better fix my odbII connection and get this thing to someone who has a reader that can monitor o2 voltage.

yota - thanks for the input but I don't have a scope. last time I hooked a scope up to a car was about 15 yrs ago

[Bubba_Jeep]

I'm pretty sure the 4-wire sensor is basically the same as a 1-wire. At least as far as measuring the varying DC voltage is concerned--signal wire to ground. Like its 3-wire cousin, it has a heater circuit that uses 2 wires--+12V and ground. In addition it has a second ground specifically for O2 signal return. The second ground insures anti seize on the sensors threads don't interfere with the signal.

[Bo185]

Originally Posted by YJ_OVER_SJ

I only have the 2 front sensors on this setup. it's in a buggy so no lights. my odbII terminal is currently on the fritz so can't run a GM tech II.

You really need to fix the DLC , then pull the codes, which I bet you have. This will tell if they are bad or not. Then buy a new O2, for which ever one is bad you ID by pulling the codes from the PCM. Easy!
Go to radio shack and buy a amber LED light, mount it near your gauges so you know if the engines throws a code so you don't fuck anything up on the motor!

[YJ_OVER_SJ]

always the pragmatist Bo. And I wanted to figure out how to bypass the PCM with a voltmeter.

yeah, need to either teach the people at kragen or autozone what a female pin on an odbII is and what it looks like OR need to go snip one off a donor at the local pick-n-pull. then I'll plug in to the PCM and do it the easy way. not as fun but easier.

[Bo185]

Originally Posted by YJ_OVER_SJ

always the pragmatist Bo. And I wanted to figure out how to bypass the PCM with a voltmeter.

yeah, need to either teach the people at kragen or autozone what a female pin on an odbII is and what it looks like OR need to go snip one off a donor at the local pick-n-pull. then I'll plug in to the PCM and do it the easy way. not as fun but easier.

Hey I keep simple, if it don't work fix it.

Just get one at a pick and pull if you need the connector itself then get new pins form radio shack and assemble.