O2 sensor voltage steady at 6.8V

Hi all well, I found the O2 sensor on the '89 300E just behind the joining of the two exhaust pipes from the manifold..... tough to get at. I've meanwhile
checked the output voltage from the sensor at the 9-pin connector on the fender. it remains 'steady' at about 6.8 V if I rev the engine it might jump around a few 10's of mV at most. Am I right to assume the O2 snesor is faulty? i was execting about 4V (70% duty cycle) and on reving would have assumed the voltage reading to jump around by a couple of volts... say from a somewhat below 4V to maybe as high as 7 or 8V..... am I on the right track here? any comments would be much appreciated.
cheers, guenter
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Last week I gave an overview of all the major sensors under the hood of an OBDII vehicle. This week let's dive into the operation of one of the most critical sensors, the Oxygen (O2) sensor, in a little more detail.
First some history. The first O2 sensor was introduced in 1976 on a Volvo. California vehicles got them next in 1980, then federal emission laws made O2 sensors virtually mandatory on all cars and light trucks built since 1981. And now that OBD-II regulations are here (1996 and newer vehicles), most vehicles now have multiple O2 sensors, some as many as four!
The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust. The signal from the O2 sensor tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).
A lot of factors affect the richness or leanness of the fuel mixture, including air temperature, engine coolant temperature, barometric pressure, throttle position, air flow and engine load. Other sensors monitor these factors too, but the O2 sensor is the master monitor for what's happening with the fuel mixture. Problems with the O2 sensor can throw the whole system out of whack.
The computer uses the oxygen sensor's input to fine tune the fuel mixture for the best balance of power, economy and emissions. The engineering term for this type of operation is "closed loop" because the computer is using the O2 sensor's input to adjust the fuel mixture. The result is a constant flip-flop back and forth from rich to lean which helps the catalytic converter operate at its best and keeps the average fuel mixture in proper balance to minimize emissions. It's a complicated setup but it works.
If no signal is received from the O2 sensor, like when a cold engine is first started (more on that in a minute) or the 02 sensor fails, the computer orders a steady, rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to fine tune the fuel mixture. If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions. As you might have guessed, that will set a code and turn on your check engine light.
How does it work? The O2 sensor produces a voltage once it gets hot. The compares how much oxygen is in the exhaust to the oxygen in outside air. The greater the difference, the higher the voltage reading.
If you ever replace an O2 sensor (and if you're a DIY'er this is something you will do eventually), its important to remember that the O2 sensor needs to "breath" outside air to work. So don't put any grease on the sensor because it could block this air flow.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen left in the exhaust. When the mixture is lean, the sensor's output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts.
When the computer reads a rich signal from the O2 sensor it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs anywhere from 2 to 7 times a second at 2500 rpm on OBDII vehicles, depending on what type of fuel injection system they have.
The oxygen sensor must be hot (about 600 degrees or higher) before it will start to generate a voltage signal. Many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly.
Ok - that was a lot of info on what they do and how they work. The next thing to know is that trouble codes relating to O2 sensors are very common. But you really need investigate further before replacing an O2 sensor just because you got that trouble code. Armed with the information above on how often the O2 sensor "flips" back and forth and AutoTap or another scantool that allows you to monitor O2 sensor voltage, you can be certain whether the O2 sensor itself is really the problem. These sensors can be pricey, so don' t just replace them the first time you see that trouble code!
I hope you found this week's topic interesting - more next week!
Happy Repairs,
Mike Fahrion www.autotap.com
Copyright 2004 B&B Electronics Manufacturing Co.
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Very informative newsgroup nugget!

Was that what was called the Lambda-Sond thing? We've had a couple of Volvos, always wondered what that was...... cp
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I have a 92 2.6 which barely passed the Cal low speed emission test for HC but was very good at the higher speed. The car did not seem to have the power it should have but appeared to run ok. A lot of troubleshooting did not turn up any issues. I stumbled on a recommendation that the O2 sensor should be replaced at 60k miles for California cars. and those without the check engine light. My vehicle had 132,000 miles on it at the time and the O2 sensor appeared to be the original factory unit. I replaced the O2 sensor and wow what a difference. Fuel economy is uip from 22 to about 26 mpg and the car certainly has more pep. If in doubt I would replace the O2 sensor in any pre OBDII vehicle. Good luck Peter
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An absolutely HUGE thank you!!!
- best regards, guenter

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