I'm diagnosing an O2 sensor fault code as mentioned in another thread. Car is a 1991 190e 2.6L with a factory installed manual transmission (717.432)

I've found a couple thorough test procedures online and have begun testing with my multimeter...But before going too far I wanted to think of any obvious causes to the problem and something came to mind.

I got the check engine light and fault-code for the o2 sensor after doing some repairs to the car. While replacing the flex discs and support bearing I pulled the entire exhaust out (leaving the manifolds attached at the head). Catalytic converter was rusted out so I cut it out and replaced it with straight pipes from the downpipe to the muffler. I also installed a new o2 sensor.

I'm assuming the o2 sensor grounds to the exhaust, and I'm now wondering if its possible I disrupted the ground. I did not notice a ground strap as I was uninstalling the exhaust.

Could I have shorted the ground to the O2 sensor? I'm assuming the ground is completed simply via the block-to-manifold-to-downpipe connection. All three are securely connected, but I'll be putting the car up on stands again to inspect.

Any input would be greatly appreciated. I'll update this thread as I conduct the tests and solve the issue, so it's useful for future visitors.


Thanks!
Roland

See sensor test comment in you first post.
A sensor may be grounded either through the shell, or, via a dedicated lead.
The shield of a coax cable may be used as the ground.
Test the sensor out-of-circuit as indicated. You have explained that you know how a O2 sensor operates, hence you will not have difficulty testing it.

Thanks Frank that's incredibly helpful. I'll test the sensor out of circuit as you suggested.
If anyone can comment on the original post regarding how specifically the o2 sensor is grounded on a 91 w201, I'd greatly appreciate it.

I'm pretty sure that is a 3 wire sensor and gets its ground though the exhaust system. Where it bolts to the manifolds should be sufficient ground assuming the engine ground is ok.

An easy test to see if the ECU is responding to the O2 input is to touch the sensor lead (disconnected) with one hand and the battery positive with the other and see if the lambda changes with it. You'll transmit just a few millivolts through your hands and simulate O2 voltage. If the lambda doesn't respond (operating temp)you could have a bad ECU, if it does respond look towards the sensor.

__________________

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3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
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You're right it is a 3 wire sensor. Thanks a bunch for the test suggestion. I'll perform that test this evening to see if the ECU is fried.

Last night I tested the o2 sensor out of circuit as the car warmed up and I thought I'd post the results here. Only problem is, I don't know the spec! Can anyone point me to them?

I only had the in-dash temp gauge to reference hence the imprecise temperatures listed below. These measurements were taken by probing the single sensor wire from the O2 sensor with my positive lead, and grounding to an unpainted door hinge bolt.

.0782V at 60c
.232V at 80c
.351V at 80c & increasing
.377V just at 80c
.380V just above 80c
.389V just above 80c
.396V just above 80c

The O2 sensor produced steadily increasing voltage as the car reached operating temp. I then blipped the throttle and voltage began to fluctuate cyclically, jumping around from 0 to ~1.9V to ~ 2-300MV. I shot a video of this but dont know how to embed on this forum so heres the link if you're interested: https://youtu.be/_XWJiZtsx3k

Last night just as I left the shop I went ahead and installed a new O2 sensor that I had lying around and also cleared the codes. I figured since I had the thing it would be an easy enough "test" to eliminate that variable.

Tonight I'll go for a drive and see if the code comes back. If it does I'll continue testing, starting by determining if the ECU is responding to the sensor.

Thanks!

I went for a drive today with the new O2 sensor installed from last night, and right around when the car gets to about 80c on the temp gauge, the check engine light comes back on. I pull over and check the codes on pin 3 and get the same 17 annoying blinks. Code 17 - oxygen sensor is shorted to positive or ground.

I remembered that I had a spare ECU that is allegedly good so I decided tonight to swap out ECUs. I also found a brand new OVP relay in the same box and put that in for good measure.

After the ECU swap I go for a drive and the check engine light comes on yet again. So again I pull over, check the codes, but this time pin 3 blinks once. No codes! I drive around the neighborhood for another 15-20 minutes and when home I check for codes in pin 3 and AGAIN I get code 17.

I've got a new O2 sensor and a "new" ECU and I'm still getting code 17 after repeatedly clearing codes. Looks like it's time to chase down this short.

I'm going to start tomorrow by directly grounding the o2 sensor temporarily with some wire to check for a weak ground through the exhaust. I know that's unlikely but I'd like to exhaust all my options before turning my attention to the wiring. I wasn't getting codes prior to dismantling the exhaust so there's a chance that something in that process is the root cause.

Any input would be greatly appreciated, as always.

Same problem,
It seems that nobody have found a solution for this...

I conducted lthe tests in the factory service manual and was able to rule out the CIS-E system as the root cause altogether. I'm now looking for a vacuum or exhaust leak but I haven't had much time to work on the car.

If you do some searching the tests are outlined on this forum and elsewhere. You'll need a multimeter capable of reading duty cycle.

I've a multimeter and I have used it several times for check the duty cycle.
I've also built a fault code reader that works very well.

Now on my C124 3,0 24v I read fault code 17 ( O2 sensor shorted to positive.....) and 9 ( current to EHA illogical....)
I changed the sensor with a new one but the fault remains...
When I try to check duty cycle I read 1,4 volts as fix value, so it's 10% in codes error index.
But this error has nothing to do with the O2 sensor.
(Sorry for my english)

Today I connect directly the O2 sensor to the ground: the fault codes 9 and 17 are always there...
I also tried to follow the green wire but at a certain point it goes somewhere with many others, it's very difficult to locate it.

Mistake...