O2 SENSOR FAILURES
O2 sensor performance can deteriorate as a result of age,
contamination or damage. This causes a decrease in reaction time
to changes in the air/fuel ratio as well as a lower voltage output.
Eventually the point may be reached where the sensor fails to
switch at all.
Symptoms of a failed O2 sensor include:
* Failed emissions test (high CO typically, but sometimes high HC too)
* Damaged catalytic converter (from an over rich fuel mixture)
* Poor fuel mileage (caused by over rich fuel mixture)
* Fouled spark plugs (caused by over rich fuel mixture)
* Runs rough
* Sluggish performance
If the average voltage from the O2 sensor is running high (more than 0.50v),
it indicates a rich condition, possibly due to a bad MAP 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, air leak at the
exhaust manifold, dirty fuel injectors, a weak fuel pump or because the O2
sensor itself is bad.
If the O2 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 elevated hydrocarbon emissions (from misfiring).
If the O2 sensor continually reads low (lean), it will cause the engine
computer to richen the fuel mixture. Injector dwell or pulse width will
increase causing fuel consumption and carbon monoxide emissions to go up.
A continually rich fuel mixture can also cause the catalytic converter to
overheat be damaged.
If the O2 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 elevated
If a heated O2 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 O2 sensor problem is not really a bad sensor. An air
leak in the intake or exhaust manifold or even a fouled spark plug or a burned
exhaust valve that allows unburned air to pass to the exhaust can cause the
O2 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 troubleshooting O2
Single wire O2 sensors are grounded through the exhaust manifold. If rust
and corrosion creates 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.
O2 SENSOR CONTAMINATION & AGING
Everybody knows that spark plugs have to be replaced periodically to
maintain peak engine performance, but many people don't realize the same
goes for oxygen sensors. As long an O2 sensor is working properly,
there's no reason to replace it. But after many miles of
being constantly bathed in hot exhaust gas, a buildup of deposits on
the sensor tip can make it sluggish. If there's enough gunk 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 emissions
soaring. The engine may also experience additional drivability problems
such as surging or hesitation.
The same kind of thing can happen if the O2 sensor has been contaminated
by deposits from sources other than normal combustion. Possible causes
of contamination include harmful fuel additives, lead, silicone (from
antifreeze leaking into a cylinder or from using the wrong type of RTV
sealer), and phosphorus from burning oil.
Oil contamination will leave dark brown deposits on the sensor tip,
while antifreeze contamination will leave light colored grainy deposits.
Black carbon deposits on the sensor tip may indicate a very rich fuel
condition. White or reddish deposits indicate fuel contamination.
Older vented O2 sensors can also be contaminated from the outside by
spilled oil, overspray of rustproofing or other chemicals (degreasers,
lubricants, etc.). Dirt or water that enters the vent hole on vented
single-wire sensors can also cause problems. Physical damage is also
possible due to the sensor's location on the exhaust manifold.
If you suspect an O2 sensor problem, the first thing you should do
is check for any codes that would implicate the sensor circuit. A code by
itself doesn't necessarily mean the O2 sensor is bad, however. It might
be a wiring problem or something else. So always check the operation of
the sensor to see if is is functioning properly before you replace it.
If there are no codes, that doesn't necessarily mean the O2 sensor
is okay. In many instances, a sluggish O2 sensor may not be bad enough
to set a fault code but will still cause an emissions or drivability
According to one California study, 70% of all fuel-injected
vehicles that failed the state's emissions test failed because they had
bad O2 sensors. Yet few of these vehicles had check engine lights or
fault codes that indicated a faulty O2 sensor.
O2 SENSOR CHECKS
Diagnostic procedures vary by vehicle manufacturer, but the following
will work on most applications:
1. First, verify basic engine health. Make sure compression, ignition,
fuel delivery and induction system are operating normally.
2. Run the engine to operating temperature and turn the engine off.
3. Disconnect the O2 sensor lead and attach the red (positive) voltmeter
test lead to the sensor signal wire, and the black (negative) test lead
to to ground.
4. Restart the engine and run it at 2,500 rpm.
5. Artificially enrich the fuel mixture by directing propane into the
intake manifold until the engine speed drops 200 rpm. The voltmeter
should rapidly show an O2 sensor output voltage of 900 mv (0.9 volts)
indicating a rich fuel mixture. No change or a low voltage output would
indicate a faulty O2 sensor.
6. Next, stop the propane and create a small vacuum leak to lean the
fuel mixture. The voltmeter should now show a drop in the O2 sensor's
output to around 200 mv (0.2 volts).
7. To check the sensor's dynamic response, turn the engine off and
reconnect the sensor lead. Then start the engine and run at 1,500 rpm.
The sensor's average output should fluctuate around 450 to 500 mv
(0.45 to 0.50 volts).
The O2 sensor's output signal can also be observed on an oscilloscope.
Leave the sensor wires connected and backprobe the connector with the
scope leads. Connect the scope lead to the sensor signal wire, and
ground to ground. A "good" waveform should show an output voltage
that changes back and forth from about 800 mv (0.8 volts) to 200 mv
(0.2 volts). The signal should respond to changed in the fuel mixture
within 300 milliseconds or less. GM says a good sensor should be able
to switch from rich to lean in less than 125 milliseconds, and from
lean to rich in less than 100 milliseconds.
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.
BENCH TESTING AN O2 SENSOR
One or two wire zirconium O2 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 wire
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.
Replacement sensors must be the same basic type as the original (heated
or unheated) and have the same performance characteristics and heater wattage
WARNING: Installing the wrong O2 sensor could affect engine performance
and possibly damage the heater control circuit in the engine computer.
Most replacement sensors have a conductive anti-seize coating on the threads
to make future replacement easier. If the threads are not coated, apply
some anti-seize to the threads. Be careful not to get any anti-seize on
the sensor shell or vented tube.
REPLACING O2 SENSORS FOR PREVENTIVE MAINTENANCE
One way to assure maximum engine performance, the lowest emissions and
best possible fuel economy is to replace aging O2 sensors before they
get too old and start causing rouble.
Bosch recommends replacing unheated 1 or 2 wire wire O2 sensors on 1976
through early 1990s applications every 30,000 to 50,000 miles.
Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications
should be changed every 60,000 miles.
On 1996 and newer OBD II equipped vehicles, the recommended replacement
interval is 100,000 miles.