An oxygen sensor is the most important sensor on a fuel injected
car. Located in the exhaust manifold of the car, its function is
to compare the difference in the oxygen content in the exhaust gas
against the oxygen content in ambient air. The results are expressed
in an analog voltage signal often referred to as Lambda. This information
is relayed to the car's engine computer (PCM) which monitors and
adjusts the air/fuel ratio accordingly. Cars produced after 1986
have at least one oxygen sensor located in the exhaust manifold
and 1996 and newer have at least two sensors, one often located
before and after the catalytic converter.
A stoichiometric or ideal air/fuel ratio is 14.7:1 which means
that there are 14.7 parts of air and one part fuel. The reaction
of this mixture would produce emissions that contained only carbon
dioxide and water, no fuel or oxygen would be left. At an air/fuel
ratio of 14:1 the oxygen sensor voltage signal would read 1.0 volt
on a narrow band sensor and 2.5 volts on a wide band sensor. If
the air/fuel ratio mixture is rich (more fuel than air) than the
sensor would read fewer volts or conversely if the mixture is lean
(more air than fuel) the O2 sensor would read a larger voltage.
If the air/fuel ratio is richer than 11.7:1 (0.8 volts) or leaner
than 18:1 (1.22 volts) the engine won't run.
There are 2 basic types of oxygen sensors on the market: narrow
band and wide band.
Narrow Band Sensors
The narrow band sensors are limited in the fact that they can only
tell if the mixture is rich or lean but not by how much. The voltage
signal varies around 1.0 volt and are really only accurate at a
14.7 air/fuel ratio, the further you deviate from this ratio the
more inaccurate the readings become.
The graph shows voltage vs. lambda. The stoichiometric value for
a narrow band O2 sensor is .5 volt indicated where the dashed black
line crosses the red solid line. This graph depicts why it is so
difficult to tune with a narrow band O2 sensor. As you can see there
is a steep rise to the left and a step fall to the right where these
two lines intersect. Moving slightly to the left or right (x-axis)
of the dashed black line produces a significant y-axis movement.
Also, the range of a narrowband sensor is only 0-1 volt which limits
the tuning resolution. If your car is running rich all you really
can tell is that you are left of where the two lines intersect.
Similarly if the car is lean you are to the right of where the lines
intersect, this is about as good as you can get with a narrowband
Saturn's are equipped from the factory with a single wire non-heated
oxygen sensor. The sensor is grounded by the exhaust manifold while
the single wire provides the PCM Lambda information.
A non-heated sensor relies on the exhaust gases to keep it at its
operating temperature which works most of the time but during extended
periods of idling or low speed operation can cause the sensor to
drop below its operating temperature and show a false lean mixture.
Oxygen sensors must warm up to at least 400 degrees Celsius before
generating an accurate signal. This is why cars remain in open loop
(preprogrammed fuel map) when the engine is cold and don't switch
to closed-loop (relying on the O2 sensor readings) until the engine
has warmed up.
3 and 4 wire sensors are heated. One wire sends Lambda information
to the PCM while the 2 additional wires are for either additional
ground wires or 12v heater voltage (delivering power whenever the
ignition is turn on). Heated sensors have several advantages over
non-heated sensors. Heated sensors warm up more quickly than non-heated
sensors and provide an accurate voltage signal regardless of the
operating conditions. Also, with turbo charged cars the turbo absorbs
so much heat that it prolongs the oxygen sensor warm-up which can
yield false sensor readings. Using a heated sensor is highly recommended
for turbo applications and in some cases less expensive than the
Wide Band Sensors
The 5-wire wide band sensor increases the Lambda resolution considerably
from 0.0 to 5.0 volts. This increase in resolution is needed to
accurately tune an engine. The graph below shows how an increase
in resolution can really make a big difference in the ability to
tune a car. Bottom line: in order to tune your car for the maximum
power and reliability you will need a wideband oxygen sensor.
*Graph taken from the Mega Squirt website.
Sometimes it is desirable to run either leaner or richer than 14.7:1.
If your goal is to produce maximal power output of the engine, as
with forced induction, you'll want to run a richer air/fuel mixture
around 12.5:1. The additional fuel in the cylinder will not burn.
For example if we added 2 parts fuel to 14.7 parts air only one
part of fuel will burn and do work. This is because there is not
enough air to burn the second part of the fuel, remember that one
part fuel combines with 14.7 parts air in a stoichiometric reaction.
This unburnt fuel takes up valuable space in the cylinder but in
forced induction applications cylinder temperatures can skyrocket
and cause detonation. This extra fuel provides an anti-knock or
cooling effect allowing for more power to be produced.
If you ran at an air/fuel ratio of 12.5:1 all the time you would
waste a lot of fuel but fortunately you don't need to. Normally
it is only necessary to enrich the fuel mixture under load i.e.
when boosting or engaging nitrous at all other times it is better
to run a little leaner than 14:1. Running a lean air/fuel ratio
will improve your gas mileage but be careful if you run too lean
you will cause your combustion chambers to overheat. Running lean
under power destroys engines because the combustion chambers get
so hot that the pistons fall apart under the stress which is mostly
due to pre-ignition. This will defiantly make for a bad day very
Testing your oxygen sensor
Warm up the engine until it has reach normal operating temperature
you will need to have the car in closed loop. Once in closed loop
the sensor voltage will vary between 0.1 and 0.9 volts. Probe the
wire with a voltmeter. If the voltage does not very and remains
fixed at about 0.2 volt then the car has not fully warmed up yet
and is still in open loop. If the voltage does vary increase and
decrease the engine speed and note the voltage change. If the voltage
fluctuates slowly, changing every 1 to 2 seconds then you have a
"lazy sensor" and it should be replaced. If the engine
has fully warmed up yet there is no change in the sensors voltage
reading then the sensor is bad and needs to be replaced. Check all
of your connections thoroughly before replacing your sensor; you
may just have a bad ground. If you do replace your oxygen sensor
it's a good idea to use antiseize on the threads in case you have
to remove it later.
forum has become one of the best resources for Saturn performance
on the net. Our members are constantly pushing their cars to new
performance levels. So, log in, share your ideas, and help push
your car’s performance to the next level.
August 2005 TSN will be holding its first ever meet
in Chardon, Ohio. Some of the fastest Saturns in the country will
be attending. Activities will include drag racing, dyno, tech session,
and BBQ. Look for the latest information in the forums.
Jeff and his team Different Racing have big plans
on breaking into some really low quarter mile times this year. Last
year he posted a 12.40 on a pretty healthy nitrous shot. This year
he has a new turbo setup and some serious determination. Visit his
website for the latest news and information. I expect we'll be seeing
some low 12's from him by the end of the year.