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Discussion Starter #1
My car narrowly passed the NYS emissions test with a very high CO reading. The car guzzles gas like nobody's busness and now the "Check Engine" light is going on sporadically. My suspicions are that the O2 sensor might be going. The car has well over 120k miles on it and as far as I can guess, the O2 sensor has never been replaced. The cost to replace is about $200-300 depending where I go. I know there is a way to test the voltage on the sensor. Does anyone have any info or schematic that would alloe me to do this. The connector(s) for the sensor are located under the passenger seat.

Thanks
 

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'85 2.3-16 '99 C280 '11 GLK350
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Using a digital voltmeter with reasonably high impedence (20K ohms or so), you can measure the voltage output of your O2 sensor while it's connected to the computer, with the car warm and running of course. Just find the single wire connector coming from the sensor that goes to the computer - not the one that has two wires, that's the preheater on some sensors. Set your meter to a range near 1 volt - the sensor's output falls in the 0-1 volt range. Ground the (-) probe to a chassis ground or the battery, and clip the (+) lead to the sensor output. You should be getting a reading in the neighborhood of 0.45 volts, fluctuating a bit up and down. If you get no voltage or a low voltage, your sensor needs replacement.

BTW, the 300E (and the 190E) uses the same Bosch sensor that's used in the BMW, Saab and Volvo. But more importantly, it's the same as the sensor for the 1990 Ford Mustang 5.0L V8 (Bosch PN 13942) which has the requisite metric threads (with a little Anti-seize already on them). The only differences between the Ford sensor and the Mercedes sensor is 1) the end connector; 2) name of the car it goes into - based on the salesman's computer database; and 3) PRICE!!!!!

If you're buying the sensor from a non-Ford parts dept (Autozone, NAPA, PepBoys, etc.) they may tell you they have more than one sensor for the Mustang application, since Mustangs had two sensors, so don't let the parts guy sell you two for your non-existent pony car.

The sensor is the three-wire type, (with two white heater wires), To fit, cut off the Ford connector and solder the existing Mercedes connector on to it (while matching wire colors), and install. .
 

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Thanks ... just so I am certain, the only way that I can think of (aside from stripping the insulation from the wire) is to diconnect the 02 sensor (connectors beneath pasgr floor cover). The 02 sensor has (2) cables ... one is a round single wire connector. The other is a (2) wire conduit that terminates into a rectangular connector. I am assuming youwere referring to the round single one. Again, I assuming that the 02 sensor works similar to a thermocoupler in that when the TC is exposed to heat, it sends a small voltage so I would guess the 02 sensor operates similarly in that, it is NOT connected to a volage source, but rather it will send a small trace voltage if it's operating correctly.

I'll try this.

Thanks again
 

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1990 190e 2.6l Calif. auto 1987 Ford F150 4.9L Calif. man 4OD 4X4 2013 Suzuki C50 Crusier
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you can stick a small safety pin in the single wire maybe ( Green ) and connect the red pos wire from your DVOM and the black meter lead to a good ground. Then start car, you should have about .400 volts ( engine in open loop ) after the engine warms up it should go into ( close loop ) at that point the meter reading should move above and below the .400v. then give it some gas and you should get a reading about .800 or more showing a rich mixture. Then let off the gas and you should get a reading of .100v or even .045v there abouts , showing very lean.
 

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1990 190e 2.6l Calif. auto 1987 Ford F150 4.9L Calif. man 4OD 4X4 2013 Suzuki C50 Crusier
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Well it can't hurt to add some info. If you look at this site you will see lots of guys that have posted problems here and don't get info that they are looking for and all of a sudden the post stops. You don't know what happen. I am sure if someone finds this post that is having a problem would be happy to find this info no matter what the date span is.
 
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