Lambda probe
An automotive lambda probe, also known as a lambda sensor, O2 sensor, oxygen sensor, lambda sond or EGO (exhaust gas oxygen) sensor, is a small sensor inserted into the exhaust system of a petrol engine to measure the volume of oxygen (O2) remaining in the exhaust gas to allow an electronic system (ECU) to control the efficiency of the combustion process in the engine.
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Function of a lambda probe
Lambda probes are used to reduce vehicle emissions, by ensuring that engines burn their fuel efficiently and cleanly. Robert Bosch GmbH introduced the first automotive lambda probe in 1976. The sensors were introduced in the US from about 1980, and were required on all models of cars in many countries in Europe in 1993.
By measuring the amount of oxygen in the remaining exhaust gases, and by knowing the volume and temperature of the air entering the cylinders amongst other things, an ECU can use look-up tables to determine the amount of fuel required to burn at the stoichiometric point (14.7:1 fuel:air by mass) to ensure complete combustion.
The probe
The sensor element is a ceramic cylinder plated inside and out with porous platinum electrodes, the whole assembly protected by a metal gauze. It operates by measuring the difference in oxygen between the exhaust gas and the external air, and generates a voltage or changes its resistance depending on the difference between the two. The sensors only work effectively when heated to approximately 300°C, so most lambda probes have heating elements encased in the ceramic to bring the ceramic tip up to temperature quickly when the exhaust is cold. The probe typically has four wires attached to it; two for the lambda output, and two for the heater power.
Operation of the probe
Zirconia sensor
The zirconium dioxide, or zirconia, lambda sensor is based on a solid-state electrochemical fuel cell called the Nernst cell. Its two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere. An output voltage of 0.2 V (200 mV) DC represents a lean mixture. That is one where the amount of oxygen entering the cylinder is insufficient to fully oxidise the carbon monoxide (CO), produced in burning the air and fuel, into carbon dioxide (CO2). A reading of 0.8 V (800 mV) DC represents a rich mixture, one which is high in unburned fuel and low in remaining oxygen. The ideal point is 0.45V (450 mV) DC; this is where the quantities of air and fuel are in the optimum ratio, called the stoichiometric point, and the exhaust output will mainly consist of fully oxidised CO2.
The voltage produced by the sensor is so nonlinear with respect to oxygen concentration that it is impractical for the electronic control unit (ECU) to measure intermediate values – it merely registers "lean" or "rich", and adjusts the fuel/air mixture to keep the output of the sensor alternating equally between these two values.
This type of sensor is called 'narrowband', referring to the narrow range of fuel/air ratios to which the sensor responds. The main disadvantage of narrowband sensors is their slow response: the control unit determines the exhaust gas composition by averaging the high and low swings in the sensor's output, and this process creates an inevitable delay.
Wideband zirconia sensor
A variation on the zirconia sensor, called the 'wideband' sensor, was introduced by Robert Bosch in 1994 but is presently (2005) used in only a few vehicles. It is based on a planar zirconia element, but also incorporates an electrochemical gas pump. An electronic circuit containing a feedback loop controls the gas pump current to keep the output of the electrochemical cell constant, so that the pump current directly indicates of the oxygen content of the exhaust gas. This sensor eliminates the averaging delay inherent in narrowband sensors, allowing the control unit to adjust the fuel delivery and ignition timing of the engine much more rapidly. In the automotive industry this sensor is also called a UEGO (for Universal Exhaust Gas Oxygen) sensor.
Titania sensor
A less common type of narrowband lambda sensor has a ceramic element made of titanium dioxide (titania). This type does not generate its own voltage, but changes its electrical resistance in response to the oxygen concentration. Its value varies from about 20 kilohm for a lean mixture to about 1 kilohm for a rich mixture. The control unit feeds the sensor with a low-current 5 volt supply and measures the resulting voltage across the sensor. Like the zirconia sensor, this type is so nonlinear that it in practice it is used simply as a binary "rich or lean" indicator. Titania sensors are more expensive than zirconia sensors, but have a faster response.
Location of the probe in a system
The probe is typically screwed into a tapped hole in the exhaust, located after the branch manifold of the exhaust system combines, and before the CAT (catalytic converter).
Common failure modes
Lambda probes have a limited lifespan. The exposed sensor element is subjected to high temperatures and this causes fatiuge over time. Probes become sluggish, failing to react quickly to changes in the condition of the exhaust gases. This can be caused by contamination by elements in 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.
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.