What does it mean?
In the internal combustion engine, the ideal chemical reaction of combustion is as follows: oxygen (O 2 ) + fuel (HC) = carbon dioxide (CO 2 ) + water (H 2 O). Unfortunately, because engines are used over such a wide range of situations, ideal combustion often difficult to achieve.
Outside of the lab, for example, there is a wide range of ambient temperature and humidity conditions, atmospheric pressure changes, as well as differing driver demands and even the state of the engine itself, all of which can lead to less-than-ideal combustion.
Modern engines employ a number of strategies to achieve ideal combustion in ever-changing circumstances, using systems like variable valve timing and lift, variable ignition timing, direct fuel injection, and exhaust gas recirculation (EGR), to name a few. The EGR system is designed to reduce the production of nitrogen oxides (NO x ), which are responsible for the formation of smog and acid rain (Nitric Acid – HNO 3 ), amongst other environmental and health hazards.
Under ideal conditions, nitrogen (N 2 ) would normally not react with the oxygen during combustion, but abnormally-high combustion temperatures, typically over 2,400 °F, can enable N 2 reactions and the formation of NO x . The EGR system injects a small amount of spent exhaust gases, routed from the exhaust pipe, back into the intake, reducing the amount of O 2 available for combustion, therefore reducing combustion temperatures and the production of NO x .
The EGR system connects the exhaust to the intake via pipes and a valve. The EGR valve used to be vacuum-modulated, but many vehicles have switched over to electronic control by the engine control module (ECM). Depending on demand, the ECM commands the EGR to open and close, controlling the flow of exhaust gas into the intake. The ECM also uses sensors to monitor system function, such as the EGR Temperature or Flow Sensor and EGR Position sensor.
If the ECM detects a problem in the EGR position sensor circuit, it will set a diagnostic trouble code (DTC) and illuminate the malfunction indicator lamp (MIL).