Diagnostic Trouble Codes – P0401, P0402, P0403, P0404, P0405, P0406

• P0401 – EGR Flow Insufficient Detected
• P0402 – EGR Flow Excessive Detected
• P0403 – EGR Control Circuits
• P0404 – EGR Control Circuit Range/Performance
• P0405 – EGR Sensor A Circuit Low
• P0406 – EGR Sensor A Circuit High

The Exhaust Gas Recirculation (EGR) system’s purpose is to redirect exhaust gas back into the cylinders. Since exhaust gas is inert (it won’t burn), it displaces oxygen and fuel, thereby lowering cylinder temperatures, which in turn lowers oxides of nitrogen emissions. For that reason, it needs to be carefully metered into the cylinders (via the EGR valve) so as not to adversely affect the engine’s performance. (Too much EGR and the engine won’t idle.)