OBD-II · Engine and powertrain (P)
P0401
EGR flow insufficient
What it means
Less exhaust gas is being recirculated than the engine computer expects - usually because the EGR path is blocked with soot.
Symptoms
- engine warning light
- often no noticeable change
- sometimes knocking or harsher combustion
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | carbon-clogged EGR valve that no longer opens fully |
| 02 | blocked EGR cooler or intake manifold |
| 03 | failed EGR actuator |
| 04 | vacuum leak or vacuum supply fault on vacuum-operated valves |
| 05 | air-mass meter reading wrong |
What to check, step by step
- 01Check live data: EGR position and air-mass change when EGR opens.
- 02Run the actuator test.
- 03Remove the valve and inspect for carbon; clean or replace.
- 04Check EGR cooler and intake manifold for soot restriction.
- 05Check vacuum lines and solenoid on vacuum-operated systems.