P0405

What P0405 Means

Code P0405 points to a low signal level from the EGR valve position sensor (Exhaust Gas Recirculation). The position sensor (Sensor A) monitors the actual position of the exhaust gas recirculation valve and sends a signal to the engine control unit (DME). With a low signal level, the DME can't determine valve position correctly, which disrupts control of the recirculation process.

The EGR system reduces nitrogen oxide emissions by routing part of the exhaust gas back into the intake manifold. The EGR valve opens and closes on command from the DME, and the position sensor provides feedback. A low signal can come from a faulty sensor, an open or short in the wiring, or a dirty or stuck valve.

The code falls under the intake_maf category and is rated moderate severity.

Symptoms

• Rough engine operation (hesitation, fluctuating rpm, especially at idle or under light load).
• The engine may drop into limp mode (power limitation, check engine light on the instrument cluster).
• Symptoms may show up on a warm engine or when the operating mode changes.

Note: the exact symptom list depends on the specific BMW model and operating conditions.

Diagnosis and Repair

1. Read the fault codes with diagnostic equipment (ISTA, INPA) and check for related EGR system errors.
2. Check the EGR sensor and its wiring: inspect the connectors for corrosion, damaged insulation, opens, or shorts. Test the circuits with a multimeter against the wiring diagram.
3. Check sensor resistance (if applicable) and compare it to the manufacturer's reference values.
4. Clean or replace the EGR valve if you find contamination or mechanical sticking.
5. If the sensor is confirmed faulty, replace the EGR valve position sensor (part number verified by the vehicle's VIN).
6. Reset the adaptations and take a test drive to confirm the code doesn't return.

Common diagnostic mistake: replacing the sensor without checking the wiring and the cleanliness of the EGR valve.