The oxygen sensor, also known as a lambda sensor, was developed in the late 1960s by Dr. Gunter Bauman for the Robert Bosch company. This sensor was made using ceramic coated zirconia and platinum. However, in order to make the O2 sensor more efficient and capable of mass production NTK, in 1990, developed planar oxygen sensors for use in the Honda Civic and Accord.
All cars that were manufactured post-1980 feature oxygen sensors. They make modern electronic fuel injection and emission control possible by determining, in real time, whether the air–fuel ratio of a combustion engine is rich or lean. The primary goal is a compromise between power, fuel economy, and emissions. The ideal ratio for oxygen and gasoline is 14.7:1, which slightly varies depending on different types of gas. The sensor, however, does not actually measure oxygen concentration, but rather the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in air.
The sensor element is a ceramic cylinder plated inside and outside with porous platinum electrodes; the whole assembly is 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. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess. The sensors only work effectively when heated to approximately 600°F, so most newer lambda probes have heating elements encased in the ceramic that bring the ceramic tip up to temperature quickly. Older probes, without heating elements, would eventually be heated by the exhaust, but there is a time lag between when the engine is started and when the components in the exhaust system come to a thermal equilibrium. The length of time required for the exhaust gases to bring the probe to temperature depends on the temperature of the ambient air and the geometry of the exhaust system. Without a heater, the process may take several minutes. There are pollution problems that are attributed to this slow start-up process, including a similar problem with the working temperature of a catalytic converter.
The probe typically has four wires attached to it: two for the lambda output, and two for the heater power, although some automakers use the metal case as ground for the sensor element signal, resulting in three wires. Earlier non-electrically-heated sensors had one or two wires. It has a rugged stainless-steel construction internally and externally. Due to this the sensor has a high resistance to corrosion, allowing it to be used effectively in aggressive environments with high temperature/pressure.
The probe is typically screwed into a threaded hole in the exhaust system, located after the branch manifold of the exhaust system combines and before the catalytic converter. New vehicles are required to have a sensor before and after the exhaust catalyst to meet U.S. regulations requiring that all emissions components be monitored for failure. Pre- and post-catalyst signals are monitored to determine catalyst efficiency, and if the converter is not performing as expected, an alert gets reported to the user through on-board diagnostic systems by lighting up an indicator in the vehicle's dashboard. Additionally, some catalyst systems require brief cycles of lean (oxygen-containing) gas to load the catalyst and promote additional oxidation reduction of undesirable exhaust components. For states that have motor vehicle inspection programs to regulate emissions, the use of the CEL and O2 light will alert officials to any excessive emissions. As a result, if one or more oxygen sensors is faulty during an emissions inspection, the vehicle will not pass the inspection.
Cars with O2 sensors have a minimum of one sensor in front of the catalytic converter, as well as one in each of the car’s exhaust manifold. The actual number of oxygen sensors for a car depends on the year, make, model and engine. While most later model vehicles have four oxygen sensors, the actual number varies by engine type:
- 4 cylinder transverse has an upstream and a downstream O2 sensor.
- V6 and V8 transverse have four oxygen sensors including a left or front bank upstream; right or rear bank upstream; rear of the engine; and a downstream sensor.
- 4 and 6 cylinders in-line have three oxygen sensors including a front and rear bank upstream and a downstream sensor.
An oxygen sensor can be tested either by leaving it attached to the vehicle or by removing it for testing. Testing requires two tools: a high-impedance digital voltmeter and a back probe. The first step to checking an O2 sensor is to locate the surrounding wires to make sure they are intact and without visible signs of wear and tear. Next, the vehicle must be started and allowed to run until the engine reaches 600 degrees F in order to ensure an accurate reading of the sensor. Using the back probe and voltmeter, the oxygen sensor is measured at a set number of points and under particular conditions to determine any faulty measurements. As the testing of an oxygen sensor requires specialized training and tools, it is best to allow a mechanic to handle this voltage-based testing.
When an engine is operating at low-load conditions (such as when accelerating very gently or maintaining a constant speed), it is in a "closed-loop mode". This refers to a feedback loop between the ECU and the oxygen sensor(s) in which the ECU adjusts the quantity of fuel and expects to see a resulting change in the response of the oxygen sensor. This loop forces the engine to operate both slightly lean and slightly rich on successive loops, as it attempts to maintain a mostly stoichiometric ratio on average. If modifications cause the engine to run moderately lean, there will be a slight increase in fuel efficiency, sometimes at the expense of increased NOx emissions, much higher exhaust gas temperatures and sometimes a slight increase in power that can quickly turn into misfires and a drastic loss of power, as well as potential engine and catalytic-converter (due to the misfires) damage, at ultra-lean air–fuel ratios. If modifications cause the engine to run rich, then there will be a slight increase in power to a point (after which the engine starts flooding from too much unburned fuel), but at the cost of decreased fuel efficiency, and an increase in unburned hydrocarbons in the exhaust, which causes overheating of the catalytic converter. Prolonged operation at rich mixtures can cause catastrophic failure of the catalytic converter. The ECU also controls the spark engine timing, along with the fuel-injector pulse width, so modifications that alter the engine to operate either too lean or too rich may result in inefficient fuel consumption whenever fuel is ignited too soon or too late in the combustion cycle.
When an internal combustion engine is under high load (e.g. wide open throttle, the output of the oxygen sensor is ignored, and the ECU automatically enriches the mixture to protect the engine, as misfires under load are much more likely to cause damage. This is referred to as an engine running in "open-loop mode". Any changes in the sensor output will be ignored in this state. In many cars (with the exception of some turbocharged models), inputs from the air flow meter are also ignored, as they might otherwise lower engine performance due to the mixture being too rich or too lean, and increase the risk of engine damage due to detonation if the mixture is too lean.
Normally, the lifetime of an unheated sensor is about 30,000 to 50,000 miles. Heated sensor lifetime is typically 100,000 miles. Failure of an unheated sensor is usually caused by the buildup of soot on the ceramic element, which lengthens its response time and may cause total loss of ability to sense oxygen. For heated sensors, normal deposits are burned off during operation, and failure occurs due to catalyst depletion. The probe then tends to report lean mixture, the ECU enriches the mixture, the exhaust gets rich with carbon monoxide and hydrocarbons, and the fuel economy worsens.
Symptoms of a failing oxygen sensor includes:
- Sensor light on dash indicates problem.
- Increased tailpipe emissions.
- Increased fuel consumption.
- Hesitation on acceleration.
- Rough idling.