Q: Does the OBD System Measure Gases in the Car's Exhaust?

The onboard diagnostic (OBD) system utilizes sensors mounted in the engine exhaust stream to measure the amount of oxygen that comes out of the engine after the combustion process. When an engine intakes a certain amount of air into the engine, the computer will inject the correct amount of fuel using its internal programming and input from several sensors, including the O2 (oxygen) sensors mounted in the exhaust manifold to test the exhaust oxygen content. The input of the O2 sensor is used to maintain a fuel-to-air ratio to be as close to stoichiometric 14.7 parts air to 1 part fuel.

The O2 sensor can measure the amount of oxygen in the exhaust stream using its internal design and the heat in the exhaust after the engine has completed the combustion process. If the combustion process was incomplete or too much fuel is injected, the O2 sensor will sense an increase in oxygen in the exhaust stream indicating the engine is not burning all the fuel. If the computer lowers the amount of fuel injected into the engine cylinders, then the O2 sensor may indicate a lower oxygen content in the exhaust. The computer will use this exhaust O2 sensor as feedback to control the fuel going into the engine.

The engine may use multiple O2 sensors to monitor the engine fuel control by having sensors on both sides of a V-style engine. This is done so dual exhaust can be utilized and monitoring of both sides of the engine can be done. Another O2 sensor can be used after a catalytic converter to measure the catalyst’s ability to burn off excessive fuel in the exhaust. This is done by comparing the before-catalyst O2 sensor and after-catalyst sensor readings.

If the engine gets a reading from an O2 sensor that indicates a low oxygen content, then the engine computer will start to lower the fuel amount injected until it gets to its programming limits or the O2 sensor starts to indicate a high oxygen content. If the computer never gets a change in the O2 sensor output after reaching its programmed limit on fuel control, then it will set a code in memory for the O2 sensor. This indicates a problem with the O2 sensor or other problems in the fuel system causing excessive fuel in the exhaust.

If the engine gets a reading from an O2 sensor that indicates too much oxygen is in the exhaust, the computer will try and compensate by adding more fuel to its programmed limits. If this limit is reached and the oxygen content is still high, then a code is set. This is due to the fact that the injection system cannot inject enough fuel or there is excessive oxygen getting into the engine unmetered. The O2 sensor may also be malfunctioning and might need to be replaced.