“Know your car” Series #3


The Oxygen Sensor



The efficiency of the engine’s use of fuel is critical to performance, economy and health of the engine and the environment.

An air/fuel ratio is said to chemically correct or stoichiometric at a 14.7:1 air/fuel ratio. The oxygen sensor (O2S) monitors the oxygen content of the exhaust gasses (in comparison to the outside air) and sends a message to the car’s computer to either add or subtract fuel from the air/fuel mixture. By constantly sensing the oxygen levels the O2S can determine how well the combustion process is working.


Where is my O2S?
The O2S is about the size of your little finger. It is a metallic device, protruding about 75mm out of the exhaust pipe with a wire entering the top. It is screwed into the exhaust system just after the junction of the headers to the exhaust pipe, immediately above the header’s flange coupling to the exhaust pipe. The wire leads to the ECU (the car’s computer).


How Does It Work?
A mixture of fuel and air enters the combustion chamber, is compressed and ignited by the spark plug. The ignition process relies on the presence of oxygen and fuel. If there is too much fuel then the air/fuel ratio is said to be rich. This results in fuel levels are high and air level’s are therefore relatively low. Low air levels mean less oxygen. This results in an incomplete burning of the fuel. Indeed, a rich mixture will greatly increase the amount of the available oxygen burned so the exhaust gas will show almost an absence of residual oxygen. Conversely, lean mixtures will leave an excess of unburned oxygen and can create high nitrogen oxide levels. Lean mixtures are dangerous to the environment and to the health of your engine.


A correct air/fuel mixture will be evident by the correct levels of unburned oxygen in the exhaust gas. The O2S is constructed of material that chemically measures the difference in the oxygen content in the ambient air compared to the oxygen in the exhaust gasses.


The O2S sensor’s has a metallic cover protecting ceramic body made of zirconium dioxide with gas permeable platinum electrodes on the surface. The chemical process occurring between the surface exposed to the exhaust gas and the surface exposed to ambient air creates an electrical current. When the residual oxygen level of a chemically correct or stoichiometric 14.7:1 air/fuel ratio is detected the electrical current is 0.45. If the mixture is rich the reading will be greater than 0.45 volts. A lean mixture will be less than 0.45 volts. The chemically generated output range of the oxygen sensor is usually between 0 and 1.1 volts with a normal operating range between 0.2 to 0.7 volts.

The voltage results from the O2S are sent to the ECU, which then regulates the amount of fuel pumped into the engine (air/fuel mix). The objective is to reach a stoichiometric mix that generates a 0.45-volt reading by the O2S. When operating correctly there is a continuous crossing of the 0.45-volt mark (the "O2 cross" mark). The effectiveness of an oxygen sensor is measured in the number of "O2 cross" counts. The more O2 cross occurrences means the better the sensor and computer control systems are working. This constant feed back, monitoring result and adjustment is known as a closed loop system because the air/fuel ratio is regulated only by the results of the O2S.


What Can Go Wrong?
A constant 0.45-volt reading shows that that the O2S is not working. This might be because it has not reached the correct operating temperature of at least 600 C, or that the sensor is malfunctioning. In such cases the ECU goes into "open loop" operation and runs a rich mixture to avoid engine damage. Poor economy, poor performance and lambda readings of less than 0.89 will be present.
Since the O2S sensor is comparing the relative ratios of oxygen in the exhaust and in the air any blockage to that reading will cause wrong readings. Fuel additives that coat the inner probe or covering of the outside in oil, engine cleaning materials, antifreeze, etc. will make comparisons unreliable.


Replacement oxygen sensors are readily available from the major automotive parts manufacturers.




Safe Journey


Rob (Techno) Spargo

Mazda MX-5 Club Victoria