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.)

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