Still working on my 1990300se
replaced o2 sensor(oem).cleared code 17.but cel come back aft 10 or so miles.
now I checked for dc volts at connection under floor mat zero volts! with system connected probing green/black wire with key on nothing now what?

PS not sure which wire I should be testing at.there is one fat green wire coming from sensor and 2 wires that are attached by connector white wire and black with green stripe wire . some one posted to test at black/G wire another forum posted to test fat green wire confused?

This car has to get smogged in march.I know it wont pass with cel on,but I can clear it easily.and it wont come on for 10miles or so I'll take it to a pass or don't pay station hopefully it wont come on during the test.the car runs really good Idles perfect smells ok just that cel comes on!
usually I'm pretty good with trouble shooting skills but have the flu bug I got at work sucks!

What should the voltage be?
It's my understanding the O2 sensor creates voltage when the engine running, but none when the engine is off.

Disconnect the sensor wire from the ECU and probe it directly when the car is hot. You should get a high voltage reading regardless (between 0.7v-0.9v). If not, the sensor might be bad.

Do you still have the old sensor?

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

no threw old one away

ok I tested at the black with green stripe key on I get 8.9vdc for about 2 sec than goes to zero

Hmm somethings wrong. What brand O2 sensor did you get? Was it spliced?

Probe both white heater wires and ground negative and see if you are getting 12v/14v when engine is cold...so you may need to test tomorrow.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

I purchased the exact replacement from amazon 80bucks.no splicing involved direct fit.I'll test some more tomorrow have to pick up wife from work.
if voltage is bad then they sold me a substandard sensor.if it test good than its either bad pre cats or plugged cat .car has 160k with original ex syst.but runs great.from what I've been reading here it would run like crap!
other thing might be running to rich or lean.there is an adjustment hole for throttle body on top of air cleaner, lambda screw.its never been touched and still has a plug over it
last resort
I like my old 78 Chevy truck its so easy to work on but not very classy like the Benz!!!