Do
you know your oxygen sensors? Oxygen sensors have been used for more
than a quarter of a century, dating back to 1980 when the first
computerized engine control systems appeared. The oxygen (O2) sensor is
part of the fuel management system. It monitors unburned oxygen in the
exhaust. The powertrain control module (PCM) uses this information to
determine if the fuel mixture is rich (too much fuel) or lean (not
enough fuel).
To provide the best performance, fuel economy and
emissions, the PCM has to constantly readjust the fuel mixture while
the engine is running. It does this by looking at the signal from the
O2 sensor(s), and then increasing or decreasing the on-time (dwell) of
the fuel injectors to control fuel delivery.
INTERNAL HEATERS
Oxygen
sensors don’t produce a signal until they are hot, so the O2 sensors in
most late model vehicles have an internal heater that starts heating up
the sensor as soon as the engine starts. Older, first-generation O2
sensors lacked this feature and took much longer to reach operating
temperature, which increased cold start emissions.
Once the sensor
is hot, a zirconia-type O2 sensor will generate a voltage signal that
can range from a few tenths of a volt up to almost a full volt. When
there is little unburned oxygen in the exhaust, the sensor usually
generates 0.8 to 0.9 volts. The PCM reads this as a “rich” signal,
shortens the duration of the fuel injector pulses to reduce fuel
delivery, and leans out the fuel mixture.
When there is a lot of
unburned oxygen in the exhaust — which may be from a lean fuel
mixture,or if the engine has a misfire or compression leak — the O2
sensor will produce a low-voltage signal (0.3 volts or less). The PCM
reads this as a “lean” signal, increases the duration of the injector
pulses, and adds fuel to enrich the fuel mixture.
A slightly
different variation on this is the titania-type O2 sensor. Used in some
older Nissan and Jeep applications, this type of sensor changes
resistance rather than producing a voltage signal.
In recent years,
the design of O2 sensors has changed. The ceramic thimble-shaped
element in zirconia-type O2 sensors has been replaced by a flat strip
ceramic “planar” style sensor element.
The basic operating principle
is still the same (the output voltage changes as O2 levels in the
exhaust change), but the new design is smaller, much more robust and
faster to reach operating temperature. You can’t see the difference
from the outside because the tip of the sensor is covered with a vented
metal shroud, but many O2 sensors from 1997 and up use the planar
design.
Another change has been the introduction of “wideband” O2
sensors, which are also called “Air/Fuel” or A/F sensors. This type of
O2 sensor also uses a flat strip ceramic element inside, but it has
extra internal circuitry that allows the sensor to measure the exhaust
air/fuel ratio with a much higher degree of precision. It can tell the
PCM the exact air/fuel ratio, not just a gross rich or lean indication
as other O2 sensors do.
CATALYTIC MONITORING
In 1996,
vehicles also began using oxygen sensors to monitor the operation of
the catalytic converter. A “downstream” O2 sensor is placed either in
or just behind the converter to monitor oxygen levels after the exhaust
had reacted with the catalyst.
If the operating efficiency of the
converter drops below a certain threshold that might cause an increase
in emissions, it sets a diagnostic trouble code (DTC) for the converter
and turns on the Check Engine Light.
First-generation O2 sensors
typically have a limited service life, and may need to be replaced for
preventive maintenance somewhere between 50,000 and 80,000 miles. O2
sensors on 1996 and newer vehicles typically have a much longer service
life of 100,000 miles-plus, and do not have to be replaced unless they
have been contaminated or damaged.
When O2 sensors get old, they can
become sluggish and slow to respond to changes in exhaust oxygen
levels. Typical symptoms include a drop in fuel economy and higher
exhaust emissions.
A bad O2 sensor should not affect engine
starting, cause a misfire (unless the spark plugs become carbon
fouled), or cause engine stalling or hesitation problems. A sluggish or
fouled O2 sensor will typically read low (lean) and cause the engine to
run rich.
O2 sensors can be fouled by silicates if an engine has an
internal coolant leak and the cooling system contains a conventional
antifreeze with silicate rust inhibitors.
The O2 sensor can also be
contaminated by phosphorus and zinc from motor oil if the engine has an
oil consumption problem (worn valve guides or piston rings).
If the
heater circuit inside the O2 sensor fails, or the sensor stops
producing a signal due to an internal failure or a wiring fault (a
loose or corroded wiring connector), it will usually set an O2 sensor
code (P0130 to P0147).
The codes can be read by plugging a scan tool
into the vehicle’s diagnostic connector. But many times, other engine
problems will set codes that may seem to indicate a bad O2 sensor, but
in fact do not.
A P0171 or P0174 lean code, for example, means the
O2 sensor is reading lean all the time. The real problem may not a bad
O2 sensor, but possibly an engine vacuum leak, low fuel pressure or
dirty fuel injectors that are causing the engine run lean. An engine
misfire, leaky exhaust valve or a leak in the exhaust manifold gasket
that allows air into the exhaust may also cause this type of code to be
set.
If an O2 sensor has failed and needs to be replaced, some
aftermarket replacement sensors require splicing the sensor wires to
accommodate all the different OEM connector styles. This type of O2
sensor provides greater coverage with fewer part numbers.
Others
come with the same style connector as the original and are easier to
install, but require many more part numbers for the same coverage.
Ever
wonder what causes O2 sensors to fail? As O2 sensors age, they slow
down. But this usually isn’t a factor until the sensor has upwards of
75,000 or more miles on it. So when an O2 sensor fails prematurely, the
cause is often contamination.
Contaminants can come from a number of
sources. If the engine has an internal coolant leak due (a crack in the
combustion chamber or a leaky head gasket), and the coolant contains
silicate corrosion inhibitors (which conventional green coolants do but
long life orange coolants such as Dex-Cool do not), the silicates can
pass into the exhaust and contaminate the O2 sensors.
Another source
of contamination is the anti-wear ingredients in ordinary motor oil.
The amount of phosphorus and zinc in motor oil has been reduced in
recent years to reduce the risk of O2 sensor and catalytic converter
contamination.
Every engine uses a small amount of oil, and over
time the contaminants can add up. As the engine accumulates miles, and
the valve guides, rings and cylinders start to wear, oil consumption
goes up. Consequently, in a high-mileage engine that is using oil,
phosphorus and zinc contamination of the O2 sensors and catalytic
converter can be a problem.
If the O2 sensors are sluggish or have
failed, they obviously need to be replaced.
But replacing the O2
sensors will only temporarily restore the fuel feedback control system.
Unless the oil burning is eliminated, the new O2 sensors will
eventually suffer the same fate. Same for a fouled catalytic converter.