Code P0405: EGR sensor 'A': signal too low

Original definition (SAE J2012): Exhaust Gas Recirculation Sensor A Circuit Low^[1][2][3][4]

• P0405Exhaust gas recirculation (EGR) sensor 'A' circuit: low signal.
• P0406Exhaust gas recirculation (EGR) sensor 'A' circuit: high signal. It is the partner of the P0405: the same sensor 'A', which according to TaT is the EGR valve position sensor, but with the signal voltage above the expected range instead of below. TaT points to a short circuit to voltage, a faulty sensor or wiring problems as typical causes.^[4][8][9]

1. 1Faulty EGR valve position sensor: it is the first cause on the list at Autodoc Club and at Mechanic Base.^[1][2][3]
2. 2Damaged wiring or a short circuit to earth between the sensor and the control unit, or the 5 V reference supply interrupted at the sensor or its connector.^{[1][2][3][10]}
3. 3Connectors that are corroded, loose or making poor contact: AutoDTCs points out that those near the hot exhaust are a frequent cause in EGR circuit codes.^{[1][2][5][6][10]}
4. 4Faulty or stuck EGR valve.^[1][2]
5. 5Soot contaminating or caking the EGR system, or wear and dirt on the valve potentiometer and on other sensors in the system.^[1][5][6]
6. 6Poor earth connection in the circuit.^[1]
7. 7Faulty control unit, an uncommon case.^[1][2]

• Engine warning light on; Mechanic Base indicates that in most cases it is the only thing you notice.^[2][10]Engine warning light: faults that cause it
• Rough idle.^[1][10]Vibrations: faults that cause it
• Loss of power and poorer response when accelerating.^[1][2]Less power: faults that cause it
• Increased fuel consumption.^[1][2]Uses more fuel: faults that cause it
• Difficulty starting when cold.^[1]Hard to start: faults that cause it
• The engine may hesitate or stall: PartsAvatar explains that, with the voltage too low, the control unit may open the EGR excessively.^[10]Engine stalls: faults that cause it
• Pinking or metallic knocking when accelerating.^[1][2][10]
• Higher emissions and a possible failure of the emissions test.^[1][2]

1. 1Read all the codes and the freeze frame data with a scan tool, clear the code and do a road test to see whether it appears again.^[2][10]
2. 2Inspect the sensor and valve connector (corrosion, loose pins) and the wiring (broken wires, chafing, especially near the hot exhaust). It is the cheapest check and a frequent cause in EGR circuit codes.^{[1][2][5][10]}
3. 3With the scan tool, command the EGR valve to open and watch in the live data the position the sensor reports: it should change smoothly as the valve moves. If the valve does not respond, disconnect the harness to check the supply.^[2][5]
4. 4With the ignition on and the engine off, use a multimeter to measure the voltage on the valve wire: it should be around 5 V, depending on the make and model. If it reads 12 V, there is a short circuit to a power supply line that needs to be located and repaired.^[2]
5. 5Check that the 5 V reference reaches the sensor, that its earth is good and that the signal responds, and compare the signals with the manufacturer's reference values: wear or dirt on the potentiometer can distort them.^[5][6][10]
6. 6Remove the valve and look for soot in it and in its passages; clean the EGR system if necessary. AutoDTCs warns that it is common to find valves that pass the electrical test but sit in front of a passage full of soot.^[1][5]
7. 7Only if the wiring, connectors and supply are fine, repair or replace the sensor, or the EGR valve with its sensor, and then check the result with the scan tool.^[1][10]