All About o2 Sensors
The exhaust gas oxygen sensor (EGO or O2), or lambda sensor, is the key
sensor in the engine fuel control feedback loop. The computer uses the O2
sensor's input to balance the fuel mixture, leaning the mixture when the sensor
reads rich and richening the mixture when the sensor reads lean.
Lambda sensors produces a voltage signal that recognises the amount of unburned oxygen in the exhaust. An
oxygen sensor is essentially a battery that generates its own voltage. When hot
(at least 250 degrees c.), the zirconium dioxide element in the sensor's tip
produces a voltage that varies according to the amount of oxygen in the exhaust
compared to the ambient oxygen level in the outside air. The greater the
difference, the higher the sensor's output voltage.
Sensor output ranges from 0.2 Volts (lean) to 0.8 Volts (rich). A
perfectly balanced or "stoichiometric" fuel mixture of 14.7 parts of
air to 1 part of fuel gives an average reading of around 0.45 Volts.
The lambda sensor's output voltage doesn't remain constant, however. It
flip-flops back and forth from rich to lean. Every time the voltage reverses
itself and goes from high to low or vice versa, it's called a "cross
count." A good O2 sensor on a
injection system should fluctuate from rich to lean about 1 per second. If the
number of cross counts is lower than this, it tells you the O2 sensor is
getting sluggish and needs to be replaced.
Most lambda sensors will cycle from rich to lean in about 50 to 100
milliseconds, and from lean to rich in 75 to 150 milliseconds. This is referred
to as the "transition" time. If the O2 sensor is taking significantly
longer to reverse readings, this too is an indication that it is getting
sluggish and may need to be replaced.
Observing the sensor's waveform on a scope is a good way to see whether or not it is slowing down with age. If the sensor
becomes sluggish, it can create hesitation problems during sudden acceleration.
Heated
Oxygen Sensors
To reduce the warm-up time of the Lambda sensor, an internal heating
element may be used. Heated O2 sensors can reach an operating temperature of as
high as 500 degrees C in as little as eight seconds! Shorter warm-up time means
the system can go into closed loop fuel control sooner, which reduces emissions
and improves fuel economy. Heating the sensor also means it can be located
further downstream from the exhaust manifold.
Titania
Oxygen Sensors
Some vehicles have a slightly different type of sensor that has a titania
element rather than zirconium. Titania O2 sensors are fitted to some Vauxhalls.
The operating principle of a titania lambda sensor is entirely
different from that of a zirconium lambda sensor. A titania lambda sensor works
like a coolant sensor. It changes resistance as the air/fuel ratio goes from
rich to lean. But instead of a gradual change, it switches very quickly from
low resistance (less than 1000 Ohms) when the mixture is rich to high
resistance (over 20,000 Ohms) when the mixture is lean.
The engine computer supplies a base reference voltage of approximately
one volt to the titania sensor, and then reads the
voltage flowing through the sensor to monitor the air/fuel ratio. When the fuel
mixture is rich, the resistance in a titania oxygen sensor is low and the
sensor's voltage signal is high. When the fuel mixture is lean, resistance
shoots up and the voltage signal drops.
Drivability
Symptoms
A lambda sensor's normal life span is 30,000 to 50,000 miles.
Common
failure modes
Lambda probes have a limited lifespan since the exposed sensor element is
subjected to high temperatures which causes fatigue
over time. Probes become sluggish, failing to react quickly to changes in the
condition of the exhaust gases. This can be caused by contamination from the
engine, fuel or additives. One common fault is silicon buildup
on the probe sensor. A common source of silicon contamination is through the use of silicone sealant in repairing water
system leaks. Lead build-up from the use of lead additives or leaded petrol will also damage the probe, as will carbon
build-up from excessive burning of engine oil. Phosphorus in the burnt oil is
also a specific damaging contaminant (but the phosphorus is part of a very
beneficial additive called ZDDP which as of 2005 is yet to be replaced with a
less 'damaging' (to the sensor or emissions equipment) yet equally effective
(in protecting the motor life) substitute. It would be expected that a probe
would last for 3 years or 40,000 miles (about 64,000 km), but it has been
observed that probes will last for up to 3 times this length. [Also, oxygen
sensors have been used successfully for 2 years of racing with leaded gas when the sensor is put into a non-leaded gas
burning vehicle to 'burn' off the lead then putting it back, etc.
As the sensor ages, it becomes sluggish. Eventually it produces an
unchanging signal or no signal at all. When this happens, the Check Engine
Light may come on, and the engine may experience drivability problems caused by
an overly rich fuel condition. Poor fuel economy, elevated CO and HC emissions,
poor idle, and/or hesitation during acceleration are typical complaints.
If the average voltage from the lambda sensor is running high (more than
0.50V), it indicates a rich condition, possibly due to a bad MAP, MAF or Air
Flow sensor or leaky injector. If the average voltage reading is running low
(less than 0.45V), the mixture is running lean
possibly due to a vacuum leak or because the sensor itself is bad.
If the lambda sensor continually reads high (rich), it will cause the
engine computer to lean out the fuel mixture in an attempt to
compensate for the rich reading. This can cause lean misfire, hesitation,
stumbling, poor idle and high hydrocarbon emissions (from misfiring).
If the lambda sensor continually reads low (lean), it will cause the
engine computer to richen the fuel mixture. Injector pulse width will increase
causing fuel consumption and carbon monoxide emissions to go up. Constant rich
fuel mixture can also cause the catalytic converter to overheat
and it may be damaged.
If the lambda sensor's output is sluggish and does not change (low cross
counts & long transition times), the engine computer will not be able to
maintain a properly balanced fuel mixture. The engine may run too rich or too
lean, depending on the operating conditions. This, in turn, may cause
drivability problems such as misfiring, surging, poor idle, and high emissions.
If a heated sensor has a faulty heating circuit or element, the sensor
can cool off at idle causing the system to go into open loop. This usually
results in a fixed, rich fuel mixture that will increase emissions.
Sometimes an apparent lambda sensor problem is not really a faulty
sensor. An air leak in the intake or exhaust manifold or even a fouled spark
plug, for example, will cause the lambda sensor to give a false lean
indication. The sensor reacts only to the presence or absence of oxygen in the
exhaust. It has no way of knowing where the extra oxygen came
from. So keep that in mind when diagnosing oxygen
sensor problems.
The lambda sensor is also grounded through the exhaust manifold. If rust
and corrosion of the manifold gaskets and bolts is
creating resistance, it may affect the sensor's output. To rule out a bad ground, use a digital volt meter
to check for a voltage drop between the sensor shell and the engine block. More
than 0.1v can cause a problem.
Lambda
Sensor Checks
A good lambda sensor should produce a fluctuating signal that changes
quickly in response to changes in the oxygen level in the exhaust. The best way
to check the sensor is to observe the sensor's output on a waveform scope or
oscilloscope. A scope will display not only the sensor's minimum and maximum
voltage readings, and average voltage reading, but also its back
and forth voltage oscillations from rich to lean.
Sensor output can also be read directly with a 10K ohm impedance digital
voltmeter, or some code readers.
CAUTION! Never use an ohmmeter on a zirconium O2 sensor in an attempt to check the sensor because doing so can
damage it. And never jump or ground the sensor's leads.
The lambda sensor's voltage reading should have a minimum reading of 200
millivolts (0.20 V) and a maximum reading of 800 millivolts (0.80 V). If the
sensor reading is averaging low (under 400 millivolts) or high (over 500
millivolts), the engine may be running rich or lean because of some other
problem.
If the sensor's output voltage never gets higher than .60v and never
drops to less than 0.30 V, it needs to be replaced. The same is true if the
sensor's output is sluggish or doesn't change.
To check the sensor's response to changing oxygen levels in the exhaust,
first create an artificially lean condition by pulling a large vacuum line.
When extra air is introduced into the engine, the sensor's voltage output
should drop to 0.2 V.
To check the sensor's rich response artificially richen the mixture by,
if possible, clamping the return fuel line momentarily. This will force more
fuel through the injectors and should cause the O2 sensor's voltage to increase
to 0.8 V.
If the sensor's output fails to respond to the changes you've created in
the oxygen level in the exhaust, it's time for a new sensor.
Zirconium sensors can also be bench tested by heating the tip with a
propane torch while monitoring the sensor's voltage output with a digital
voltmeter. Connect the positive voltmeter lead to the signal wire (normally black) coming out of the
O2 sensor and the negative voltmeter lead to the
sensor's outer shell. Then heat the tip of the sensor with the propane torch.
The tip should be hot enough to turn cherry red, and the flame must enter the
opening into the sensor tip. If you get a voltmeter reading above 600
millivolts (0.6 Volts), and the reading quickly changes as you move the flame
back and forth over the tip, the sensor is okay. A low reading or one that is
slow to change means the sensor needs to be replaced.
Removing
Lambda Sensors
Removing the sensor when the engine is cold will lessen the odds of
stripping the threads in the exhaust manifold. Penetrating oil may be needed to
loosen rusted threads. Once the sensor has been removed, the threads in the
manifold should be cleaned before the new sensor is installed. Apply graphite
grease to the sensor threads unless the threads are precoated.
Replacing
Lambda Sensors
Everybody knows that spark plugs have to be
replaced periodically to maintain peak engine performance, but many people
don't realise the same goes for oxygen sensors. As
long the lambda sensor is working properly, there's no reason to replace it.
But after 30,000 to 50,000 miles of being constantly bathed in hot exhaust gas,
a build up of deposits on the sensor tip can make it
sluggish. If there's enough clinker on the sensor tip, the sensor may produce
little or no voltage at all. This produces a false "lean" signal that
makes the computer think the engine needs more fuel, which it doesn't but gets anyway. This creates a rich fuel condition that kills
fuel economy and sends carbon monoxide and hydrocarbon emissions soaring. The
engine may also experience additional drivability problems such as surging or
hesitation.
The same kind of thing can happen if the lambda sensor is contaminated by
deposits from sources other than normal combustion. It only takes a couple of tankfull's of leaded petrol to ruin an
lambda sensor (and catalytic converter). A lead contaminated oxygen sensor will
typically have light rust coloured
deposits on the tip. Another source of sensor contamination can come from
silicone poisoning. If somebody used the wrong kind of silicon sealer to seal
up a leaky rocker box cover or manifold gasket, silicone can
find its way into the engine and foul the sensor. Silicates, which are used as
corrosion inhibitors in antifreeze, can also cause the same kind of poisoning.
Sources here might include a leaky head gasket or racks in the combustion
chamber. Silicone deposits on the sensor tip will have a shiny white to grainy
light gray appearance.
If the engine has an oil consumption problem due to worn valve stem
seals, piston rings and/or cylinders, a build up of
heavy black to dark brown oily deposits on the sensor tip can make it sluggish.
If the deposits have a black powdery consistency, the fuel mixture is running
rich. This may be due to the sensor already having failed, or it might be due
to something else such as a leaky injector or a computer problem, or constant
short journeys where the cold start system doesn't have time to come off (open
loop) known as housewives car.
When ever you
suspect a lambda sensor problem, the first thing you should do is scan for any
codes that would implicate the sensor circuit. A code by itself doesn't
necessarily mean the sensor is bad, however. It might be a wiring problem or
something else. So always follow through with the diagnostic check to diagnose
what's wrong before you replace anything.
If you don't find any codes, that doesn't necessarily mean the lambda
sensor is okay. In many instances, a sluggish sensor may not be bad enough to
record a fault code but will still be causing an emissions
or drivability problem.
This application note was kindly donated by Mobile Tuning Sunderland