P0400 - Exhaust Gas Recirculation (EGR) 'A' Flow
System
Powertrain (Engine & Gearbox)
Technical Description
The control module has detected a fault in the Exhaust Gas Recirculation (EGR) 'A' Flow (P0400).
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Exhaust Gas Recirculation (EGR) 'A' Flow – Coil Resistance | Resistance (Ω) | 3 - 30 Ω (typical) | Connector disconnected at 20°C |
| Exhaust Gas Recirculation (EGR) 'A' Flow – Duty Cycle | PWM (%) | 5 - 95 % (modulates with target) | Engine running, actuator in control |
| Exhaust Gas Recirculation (EGR) 'A' Flow – Supply Voltage | Voltage (V) | 11.8 - 14.4 V | Ignition ON at connector |
Connector disconnected at 20°C
Engine running, actuator in control
Ignition ON at connector
Oscilloscope Waveform Analysis
2 ms/div, 20 V/divPossible Causes
Channel A:Verify dedicated wiring pins and signal pair for circuit A against the schematic.
Chafed or pinched wiring harness along the signal path to Exhaust Gas Recirculation (EGR) 'A' Flow
Internal mechanical wear or carbon deposits inside Exhaust Gas Recirculation (EGR) 'A' Flow
Defective internal output driver or reference circuit in the control module for Exhaust Gas Recirculation (EGR) 'A' Flow
Symptoms
Recommended Solution
Compare signal voltage and resistance between circuit A and companion circuit B at module connector.
Review freeze-frame environmental data and perform fundamental voltage-drop and resistance checks on the circuit.
Verify loaded supply voltage and harness terminal pins at Exhaust Gas Recirculation (EGR) 'A' Flow.
Check circuit continuity and ground-side voltage drop (< 0.05 V) for Exhaust Gas Recirculation (EGR) 'A' Flow.
Check supply and the low-side driver at the ECM connector. Replace the actuator or repair the circuit based on the readings.
Mechanic's Tip
Use the scan-tool output test with the scope on the control side at the same time. If duty changes but the actuator does not move, the valve is stuck — not the electrical fault you were chasing.