A great read here for o2 sensors.
http://www.bosch.com.au/content/languag ... tion_A.pdf
At the back of the PDF there is a part number reference and a replacement guide for each sensor.
Page 11
Simplified Test Procedure
Locate the oxygen sensor and determine the wiring layout. On heated sensors check for heater element continuity, adequate 12 volt supply to
heater and correct ground when the engine is running.
Note - Many late model vehicles have the ground circuit of the oxygen sensor heating element controlled by the engine management ECU for
sensor temperature control purposes. Do not supply direct voltage or external ground to these circuits.
With engine speed at ~ 2000 RPM the sensor voltage should be seen to cycle smoothly between 100 – 900 mV (0.1 – 0.9 Volts) approximately
eight times or more every ten seconds as shown.
A contaminated or tired sensor will be slow to cycle between the sensor voltage limits and may not generate the full potential of 900 mV. It may
also be noticed that the lean swing may drop to zero voltage.
Remember!! – The oxygen sensor is a battery, a sensor that generates 900 mV constantly is generally not faulty. Faulty sensors, like a failing battery,
will be slow to cycle between the control limits and be generally slower to react to mixture changes.
Page 12
Whilst there are many factors that will contribute to accelerated oxygen
sensor failure, it should be remembered that an oxygen sensor is a
wearing part with a specific service life not unlike a platinum spark
plug. T
he oxygen sensor should have a service life ranging from 50,000
Km - 160,000 Km dependant on sensor design, however this can be
dramatically reduced by various conditions including overheating of
the sensor, chemical poisoning and impact damage.
Excluding physical damage, the majority of these conditions will result
in the failure of the ceramic thimble by affecting its porous nature.
This will result in a sensor that is slow to react to mixture change as
shown below. A slow sensor will tend to make the air / fuel ratio of the
vehicle drift rich.
Sensor Voltage Output
Normal Sensor - Voltage “cycles” between 100 & 900 mV. The average
output from the Oxygen Sensor will be ~ 450mV.
Slow, Contaminated or “Tired’ Sensor
Voltage slowly builds up and then rapidly drops off. The effect is that
the average will drop causing a rich condition.
Important facts about Oxygen Sensors.
Oxygen Sensor life spans will vary between vehicle and sensor designs,
and are effected by many factors including fuel quality and vehicle
operational characteristics.
Oxygen Sensors should be checked and/or replaced at -
50,000 km for single and two wire sensors.
80,000 km for three or four wire heated sensors.
160,000 km for planar type sensors
Oxygen Sensors can be contaminated in many ways including -
Lead fouling from incorrect fuel.
Severe carbon/oil fouling from engine/emission control defects.
Contamination from silicon products.
Thermal stress damage [fracturing of the ceramic thimble] from
excessive water in the exhaust, ie- blown head gasket.
Contamination of the Oxygen Sensor basically results in the coating
of the platinum electrodes and therefore insulates them.
**NOTE** - Oxygen Sensors cannot be cleaned!!
Oxygen Sensors will die of “old age”, they are a wearing service
part like a platinum spark plug.
As the Oxygen Sensor deteriorates over time, or is contaminated
the output from the sensor “slows down”. This causes the “average”
that the fuel management system calculates to reduce.
The lower average gives the impression that the engine is lean and
the fuel management system overcompensates to rich.
Due to the fact that the engine will operate in a rich condition at
all times, fuel consumption is naturally higher
