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.
You might find this of interest
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
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!
Copyright 2004 B&B Electronics Manufacturing Co.
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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.
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