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960 What Voltage Should Rear O2 Sensor Read? '97 960

Wren

Wren

Porkchop Sandwiches
Joined
Jun 29, 2006
Location
Athens, GA
On my scanner, my rear O2 sensor is reading 0 volts consistently. From what I've read, the front sensor should fluctuate between .2V and .8V or so, which it is doing. However, regarding the rear sensor, I've come across some conflicting information. I've read that it should be a steady ~.4V but I've also read that it should be 0V unless something is out of whack with the Catalytic converter.

I've got a constant PO172 code; the check engine light comes on occasionally but then goes out. From what I've been reading, that is from a vacuum leak of some sort, although I can't find one.

I'm thinking that I've got a dead rear O2 sensor. I checked the connector and everything seems to be OK. The car runs fine and gets good gas mileage.

What sort of voltage should I be seeing from the rear sensor? If the sensor is dead, would that give the engine code that I'm seeing? Thanks!
 
The down stream sensor provides the same type of wave form as the up stream only perhaps slightly dampened. The two signals are compared by the EEC to determine the effectiveness of the catalytic converter and can affect the rich/lean management to keep the converter at a proper operating temperature. Voltage should range from the 0.20v to 0.80v on a scope but the transitions are too fast to see with a volt meter.
 
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I can see the fluctuations on the front sensor with my scanner, but the rear sensor just shows 0 V. It may be time for a new one.
 
Wren test the rear sensor before the cat is warm enough to have lit off. Before the cat lights off it should mirror the readings of the front sensor...
Once the cat lights off it will show much lower voltage; still making a sine wave as the engine management keeps it in closed loop.
 
JohnLane said:
Wren test the rear sensor before the cat is warm enough to have lit off. Before the cat lights off it should mirror the readings of the front sensor...
Once the cat lights off it will show much lower voltage; still making a sine wave as the engine management keeps it in closed loop.
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This. The only purpose of the rear O2 is to monitor funcionality of the catalytic convertor. It won't throw any codes not related to the catalytic convertor if it's not functioning.
 
JohnLane said:
Wren test the rear sensor before the cat is warm enough to have lit off. Before the cat lights off it should mirror the readings of the front sensor...
Once the cat lights off it will show much lower voltage; still making a sine wave as the engine management keeps it in closed loop.
Click to expand...

John, I let the engine cool this afternoon enough so the coolant temperature read 95-degrees and when I started the car, the front sensor started out at .2V and climbed steadily up to .8V and then it began to fluctuate. All of this time, the rear O2 sensor registered 0 volts.
 
DVOM's are not really quick enough to monitor o2 voltages. You are best to scope them.

However, did you try creating a vacuum leak to see if the post cat o2 sensor voltage changes? Did you try unplugging the sensor?
 
Wren put that rear sensor in the front position and retest. If it makes normal voltage straight away it is good. Otherwise it is dead.

If it is good the cat works!

Beautiful sunrise this morning.
 
Just want to remind some of you of something that may be real obvious, but not mentioned in this thread. Oxygen sensors that far back from the motor need their heaters. First thing to check when you have one that seems non-responsive is the power supply to their heaters. So much easier than swapping parts with 8-foot wiring harnesses. Check the heater is powered, and then check the resistance of the heater immediately after disconnecting, so you see the fall (not rise) in resistance that can only accompany a working heater cooling down. How many sensors do you think have been replaced because of a dirty or open heater fuse.
 
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cleanflametrap said:
Just want to remind some of you of something that may be real obvious, but not mentioned in this thread. Oxygen sensors that far back from the motor need their heaters. First thing to check when you have one that seems non-responsive is the power supply to their heaters. So much easier than swapping parts with 8-foot wiring harnesses. Check the heater is powered, and then check the resistance of the heater immediately after disconnecting, so you see the rise in resistance that can only accompany a working heater cooling down. How many sensors do you think have been replaced because of a dirty or open heater fuse.
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An OBD2 car will throw an O2 heater circuit code though, even on the post-cat O2. Probably a P0141 (heater circuit failure, bank 1 sensor 2.)
 
cleanflametrap said:
Just want to remind some of you of something that may be real obvious, but not mentioned in this thread. Oxygen sensors that far back from the motor need their heaters. First thing to check when you have one that seems non-responsive is the power supply to their heaters. So much easier than swapping parts with 8-foot wiring harnesses. Check the heater is powered, and then check the resistance of the heater immediately after disconnecting, so you see the rise in resistance that can only accompany a working heater cooling down. How many sensors do you think have been replaced because of a dirty or open heater fuse.
Click to expand...

Plenty. It's still a worthwhile test even on an obd2 car. I confess I usually dot do this test even though it is the "right" way and usually dive for the fuse box or check the plug first but your test is a much better 1-step test.

Correct me if I'm wrong, but don't most metal conductors usually lower in resistance when cooling?

Theoretically, what was taught to me is what John lane was describing....front and rear more or less mirror each other (very slightly delayed at the rear) with the rear settling more as the cat warms up.
 
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Kjets On a Plane said:
Plenty. It's still a worthwhile test even on an obd2 car. I confess I usually dot do this test even though it is the "right" way and usually dive for the fuse box or check the plug first but your test is a much better 1-step test.

Correct me if I'm wrong, but don't most metal conductors usually lower in resistance when cooling?

Theoretically, what was taught to me is what John lane was describing....front and rear more or less mirror each other (very slightly delayed at the rear) with the rear settling more as the cat warms up.
Click to expand...

I have no business in this thread -- never owned a car new enough to have OBD-II or a post-cat sensor. Just saw the heater thing bite someone recently in a thread on a 240 car and tried to extrapolate on that, which is usually a bad idea. And yes, most metals do exhibit a positive temperature coefficient of electrical resistance including the heater in the oxygen sensor. I just remembered the meter backward. It should fall -- not rise -- from about 13 ohms hot to maybe 4 ohms cold depending on how quickly you get the connector apart and your ohmmeter on it. I'll go edit my goof.:oops:
 
dreamer1uk said:
DVOM's are not really quick enough to monitor o2 voltages. You are best to scope them.

However, did you try creating a vacuum leak to see if the post cat o2 sensor voltage changes? Did you try unplugging the sensor?
Click to expand...

My scanner is reading the front O2 sensor OK, it's the back on that shows no change.

I haven't tried the other two suggestions, but will.

Update: A buddy came by my shop today with his '98 S90 and his rear O2 sensor is acting exactly like mine. I'm beginning to wonder if I'm chasing a non-existent problem.

John, is there any danger in swapping the leads for the two sensors? They are different part numbers and I sure as hell don't want to fry anything, especially if everything is working as it should be.

Thanks for all of the input, guys. This is an interesting thread.
 
If you have a vacuum leak, why are you looking at the rear O2? As stated, if there's a problem with the rear O2 - you'll usually get a trouble code stating so. One of the many beauties of OBD2 diagnostics.

EDIT: http://www.obd-codes.com/trouble_codes/

As you can see, there are a couple hundred O2 sensor related possible codes, although I'm sure most aren't used on an early adoption system. Are there 2 sensors, or 4? Just curious if it separates bank 1 & 2 for fueling purposes. Have you checked for vacuum leaks? Smoke test? no cracked air boot after the MAF?? Oil consumption??
 
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There's probably not a problem with the rear O2 sensor. I was just attempting to get an answer about that the O2 sensor should be showing on my scanner after reading numerous places that there should be a voltage output from it.

There are 2 sensors. One before the cat, and one after.

It is a straight 6 engine. There is no bank 2, only bank 1.

I have checked for vacuum leaks, but haven't smoked it. No cracked boot and no oil consumption.
 
Wren said:
There's probably not a problem with the rear O2 sensor. I was just attempting to get an answer about that the O2 sensor should be showing on my scanner after reading numerous places that there should be a voltage output from it.

There are 2 sensors. One before the cat, and one after.

It is a straight 6 engine. There is no bank 2, only bank 1.

I have checked for vacuum leaks, but haven't smoked it. No cracked boot and no oil consumption.
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Lots of straight sixes consider bank 1 cylinders 1-3, and bank 2 cylinders 4-6. All the OBD2 BMW's do at least, and have 4 O2's total. 2 front, and 2 rear. But, they also have true dual exhaust, and 2 separate cats as well.
 
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