What does it mean?
OBD II fault code P0401 is a generic code that is defined as “ EGR Insufficient Flow Detected”, and is set when the PCM (Powertrain Control Module) detects a lower than expected flow of exhaust gas through the EGR (Exhaust Gas Recirculation) system. Note that this code does not apply to applications that use VVT (Variable Valve Timing) or VCT (Variable Camshaft Timing) systems, since these systems achieve the same combustion temperature reducing effect as exhaust gas recirculation.
The primary purpose of an EGR system is to introduce controlled quantities of exhaust gas into the cylinders to reduce, or eliminate the formation of harmful oxides of nitrogen, which occurs at combustion temperatures of around 1500 degrees C (2800 degrees F). In a fully functional system, the controlled introduction of exhaust gas reduces combustion temperatures to below this point.
However, the introduction of exhaust gas only happens when very specific operating conditions such as predefined engine temperature and engine speed have been met, but note that these conditions vary between applications. NOTE: Typically, exhaust gas is not introduced during idling or under hard acceleration, since the introduced exhaust gas dilutes the air/fuel mixture.
Thus, when exhaust gas is introduced into engine at the wrong time (or in the wrong quantities) combustion of the air/fuel mixture can be affected to the point where the quality of the engine idle and/or power delivery can suffer greatly. When all required conditions have been met and the PCM opens the EGR valve, the manifold pressure should change by an amount that corresponds directly with the degree to which the EGR valve is opened.
When this happens, the PCM makes suitable adjustments to fuel delivery, ignition timing, and other settings to ensure that the engine operates at optimum levels when exhaust gas is being introduced to counter the dilution of the air/fuel mixture. Therefore, when the change in manifold pressure does not agree with the amount the PCM expects to see, the PCM interprets the condition as “Insufficient Flow”, and it will set code P0140 as a result.
Depending on the application, a warning light may be illuminated on the first failure, or only after a preset number of failure cycles had occurred.