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Old 01-25-2015, 02:41 PM   #1
Metallman56
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Testing O2 Sensor Voltage Help Needed

okay so as some of you may know I've had some issues with a misfire and I think its from my o2 sensor. the reason I think this is because when I had it tuned it was not reading any voltage at the o2 sensor. so I replaced the sensor and got the same code/issue. so I decided to hook a voltage meter to the harness. I don't know what the voltage is supposed to be so I need help. the following pics are the voltage for wires a-d in order

A


B


C


D


this is from the plug that the o2 plugs into going to the pcm. anyone know what the voltage is supposed to be. or if someone wants to test it on their car for a bench make that would be awesome. car was running. thanks.
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Old 01-25-2015, 04:34 PM   #2
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AaronGTR has made plenty of valid pointsAaronGTR has made plenty of valid points
First one is the O2 sensor signal return. Basically the ground to the PCM, and what it actually uses for the signal. Second one is the O2 sensor signal feed, or the positive side. The sensor usually operates with a fluctuating signal from 100 mV to 900 mV iirc. That is showing 210 mV which would be low for running, but I don't know what it starts out at before you start the car. Maybe it should start out at 500 mV, but I have no idea? The manual doesn't say. I'd have to go out and test mine.

Third one is the main ground so you won't see any power from that. Fourth one is the ignition feed, or main power, which is why you are seeing 15+ volts. Kinda higher really... I only ever see around 14.6 on my car, but shouldn't cause a problem. Both those are for the O2 sensor heater.

What specific code was it setting? If the operating voltage stays below 300 mV it will set DTC P0131 for low circuit voltage on sensor 1. It's just weird that you weren't getting any reading at all. Even if it's low, the PCM should be seeing something. I'd suspect a break or a short somewhere in the signal supply or return wires. Something preventing power from getting to the PCM input, or grounding out the signal and making it weak. That or a bad PCM itself.

I'd take the meter and check for continuity between the sensor plug and the PCM. Get some extension wires if needed or an extra set of hand. The first port you tested is the signal low side (return). That's port A and should be a tan wire. It goes to pin #29 on PCM plug C1 (the blue one). Hook it to those two ends and check for a tone. Wiggle the harness too in different places and see if the sound crackles. Do the same thing for the second port you tested. That is B and is the signal high side (supply). Should be a purple wire, and goes to pin #10 on PCM plug C2 (the clear one).
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Old 01-25-2015, 05:05 PM   #3
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D is heater power; C is heater low (or control, PCM pin 37). D should show system voltage; C should show near zero in operation (the heater element, as the load in the circuit, should drop nearly all the voltage. B is signal, A is signal low. After reaching operating temperature, the sensor develops a voltage over the range near zero (lean) to near 1.0v (rich). Tests should be completed with the sensor connected to the harness, engine running at normal operating temperature, by back probing the connector. Accurately observing the signal with a DVOM is next to impossible, as, if all is operating properly, the oxygen sensor signal will vary in a pattern resemblant of a sine wave as the PCM chases mixture back and forth across stoich in response to the oxygen sensor's signal -- the DVOM is averaging the actual readings; an oscilloscope is ideal. Think of the pattern as a drunk bouncing back and forth off the guardrails of an icy bridge. If the meter is back probed into the signal terminals, a rudimentary check of its function can be guessed by opening a vacuum leak to see if the reading goes low, and artificially enriching (carb clean or similar) to see if the reading goes high.

I'm with Aaron - check continuity of the relevant wires between the harness connector and the PCM before getting too wrapped up with analyzing voltages.
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Old 01-25-2015, 07:01 PM   #4
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I officially am lost trying to follow what you guys are saying. I fell like billy Madison when he's trying to learn Spanish and the Spanish teacher is talking really fast and he just says "slow down''.

with that being said I appreciate the help. the code I'm throwing p0134.

what I'm picking up is both of you think I should follow the tan wire to the pcm and check for voltage?
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Old 01-25-2015, 07:51 PM   #5
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134 is an inactivity code (the PCM sees no variation below 408mV or above 512 mV for a 30 second period after control goes closed loop). This code can set if the PCM tries to go closed loop before the oxygen sensor reaches 600 deg F (sensor provides no feedback before 600 F), but this condition *should* only arise if the sensor's heater circuit has a fault, which *should* set a dedicated code.

Assuming the replacement sensor is the correct part number and is not faulty, complete the following tests:
Continuity from harness connector B to pin 10 of PCM connector C2 (ppl wire)
Continuity from harness connector A to pin 12 of PCM connector C1 (tan wire)
System voltage present on D (brn wire)
Continuity from harness connector C to pin 37 of PCM connector C2 (blk/wht wire)

Exhaust leaks within several inches of the sensor can set false inactivity codes (exhaust pulses can pull fresh air into the exhaust). Lousy PCM grounds can cause inactivity codes (a signal with an operational range < 1V is very susceptible to voltage drops). Remove the sensor, clean the threads in the pipe with the appropriate tap - inspect the sensor tip for contamination with antiseize compound. Perhaps clean a spot on the exhaust pipe and ground it to batt neg (ungrounded/poorly grounded exhaust systems can sometimes cause issues like this). Is this car running headers? The extension harness may be pin'd improperly (wasted an hour figuring that one out once...).

BEAR IN MIND: misfiring cylinder(s) discharge oxygen into the exhaust stream (if the fuel isn't burning, the air isn't either), which can play havoc with O2 signals. Normally, a misfire DTC would be set. P0134 *is* the *only* DTC set, correct? What DTCs were deactivated when the PCM was tuned?

If the fault is not eliminated with all the above hassle, a PCM problem may exist -- in your case, a broken tune.
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Old 01-26-2015, 05:36 AM   #6
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To answer your questions

I do have headers

I do have a extension harness do to the o2 sensor on the headers being further down the pipe

No exhaust leak to report

I will also say this. When i started my car with the o2 unplugged it ran fine for about 30 seconds then it started to miss/hesitate and just sound like crap. Also the exhaust smelt funny. Kind of had a rotten egg smell to it, but it wasn't overwhelming. Its kinda hard to explain. Also when i started it this morning it was running like crap with the o2 plugged back in, but then it ran fine when i was driving to work. That makes me think the pcm is getting some sort of reading
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Old 01-26-2015, 03:23 PM   #7
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Quote:
Originally Posted by Metallman56 View Post
To answer your questions

I do have headers

I do have a extension harness do to the o2 sensor on the headers being further down the pipe

When i started my car with the o2 unplugged it ran fine for about 30 seconds then it started to miss/hesitate and just sound like crap.
If this problem started after any work that may have involved installing or disconnecting/reconnecting that extension harness, it definitely deserves a close inspection.
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I'm pretty sure you don't need 600HP to merge safely onto a freeway. and if you do, you're doing it wrong.
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Old 01-26-2015, 08:36 PM   #8
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AaronGTR has made plenty of valid pointsAaronGTR has made plenty of valid points
Yeah, I ditched my extension harness long ago. I pulled the O2 sensor wires out of the main harness a little further back so I have a little more length to play with, and the aftermarket sensor I'm using has a tad bit longer lead on it than the factory one did, so now I can make it reach without the extension harness. The less connections the better. Less chance of a malfunction.
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Old 01-28-2015, 03:29 PM   #9
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the miss just started on its own a couple of years ago. it would miss basically on cold (below 40 degrees) engine starts, but would go away after a few seconds. it would also miss when the engine was under a load like going up a hill. more recently I don't know wtf its doing. once it shifts into 3rd the rpms surge and drop over and over. sometimes worse then others, and its random when it does it. if I keep my foot in the exact same position, usually when I'm accelerating, it will continue to surge and drop. if I give it more power it will stop, if I let my foot off the gas, it will stop.

I don't think its my o2 sensor now because once I plugged it back in the car ran a lot better. back to the drawing board I suppose. I just need to sell this damn thing and move on.
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Old 10-10-2020, 11:55 AM   #10
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I have same problem but Ill add a few things.

I did a continuity Test on the A + B & signal wires they reach the ECM.
I did a volt check+ heach check on C+D. O2 Sensor gets warm(slowly) & Voltage checks out

I got a ODB Voltage check on the O2 Sensor 1 it flatt lines at 420mV very small fluctuation. maybe .001

The 2nd sensor goes up to 900mv then drops to 0 if I hit the gas.


I hate to think its ECM problem, but been thru 2 O2 Sensors & everything seems ok from the O2 Sensor to the Plug All the way to the ECM.
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