What does it mean?
OBD II fault code P0409 is a generic code that is defined as “Exhaust gas recirculation (EGR) sensor A circuit malfunction”, and is set when the PCM (Powertrain Control Module) detects a general failure of, or malfunction in, the electrical control circuit of the EGR (Exhaust Gas Recirculation) system.
Note that “circuit A” refers to a particular part of the EGR control circuit, which in the case of code P0409, can typically be either the control/signal EGR temperature sensor, or, the control/signal circuit of the EGR valve’s position sensor. NOTE: Be aware that since car manufacturers do not label circuits, sensors, parts, and components in the same way, it is necessary to refer to the manual for the affected application to determine which sensor/circuit is labelled “A” on that application.
Failure to identify parts, sensors, and circuits correctly could lead to a misdiagnosis, and the unnecessary replacement of parts and components. The purpose of exhaust gas recirculation is to introduce precisely metered amounts of exhaust gas under specific conditions into the engine to reduce combustion temperatures to below 2 500 0 F, which is the temperature at which NOx (oxides of nitrogen, an atmospheric pollutant) forms as a by-product of the combustion process.
Note that since the introduction of exhaust gas into the engine has a negative impact on the efficiency of the combustion process, the EGR system is designed in such a way that only small volumes of exhaust gas is introduced at any one time, and then only enough to have a slight quenching effect on the combustion process.
While design specifics vary greatly between applications, all EGR systems consist of a valve (that is connected directly to the exhaust system), an electrically controlled solenoid or vacuum actuator to open and close the EGR valve, wiring, various sensors including flow, temperature, and sometimes differential pressure sensors, and a position sensor that communicates the position/status of the EGR valve pintle to the PCM.
In terms of operation, the PCM collects input data from a variety of engine sensors to determine when to introduce exhaust to limit the formation of NOx. Note however, that in a fully functional EGR system, exhaust gas will never be introduced when the engine is idling, at low engine speeds, or under WOT (Wide Open Throttle) conditions on a gasoline engine, since under these conditions the introduction of exhaust gas can seriously affect the combustion process.
NOTE: Since diesel engines always run with excess air, these engines are not as sensitive to the introduction of exhaust gas as gasoline engines. Therefore, the conditions under which exhaust gas is injected into a diesel engine are different from those on gasoline engines.
Nonetheless, when the PCM determines that operating conditions are such that combustion will not be affected (typically at steady cruising speeds), it will command the EGR valve to open, with the degree of opening depending on current operating conditions.
When the EGR valve opens, the position of the EGR valve pintle is monitored by a dedicated position sensor, and on most applications, the PCM interprets the position of the valve position as a volume of exhaust gas that is flowing through the EGR valve.
In this manner, the PCM is able to control both the volume and flow rate of the exhaust gas that enters the engine, but only if- all the components, including all relevant sensors and signal circuits of the EGR valve’s control system work as intended the EGR valve itself and/or all the passage through which exhaust gas flows are clear and free of restrictions i.e., not clogged up with carbon the PCM receives accurate input data on the position of the EGR valve pintle and other parameters such as (among others),accurate input data from the NOx sensors in the exhaust system From the above it should be obvious that for the EGR system to work as intended, all of the sensors, actuators, and other parts of the EGR sy