This code usually needs a check soon to find out what is wrong.
Trouble codes · P0401
P0401 meaning
EGR A Flow Insufficient Detected
The same first verdict Car Assist shows from this code alone, before a diagnosis. The code does not tell the app whether the warning light is flashing.
What this code doesn’t tell you
A code says where the car noticed a problem. It does not prove which part failed, whether the fault is still present, or what repair this car needs. The engine, other codes, freeze frame and live readings matter.
Usual causes, most likely first
The order is a starting point across cars; the exact engine and its readings can change it.
- Carbon restricting the EGR passages
- EGR valve stuck closed
- EGR flow sensor or vacuum control fault
What you can check
Note whether power or idle changed, and keep other stored codes. With the engine off and cold, look for a disconnected control hose from above; internal cleaning needs a vehicle-specific procedure.
Words the car uses
- Stored codes
- Faults the car has confirmed and saved. They help explain a check-engine light, but a code alone does not prove which part needs replacing.
- Freeze frame
- A snapshot of the engine's readings when a fault was recorded. It helps show the conditions that caused the fault, even if the car now seems fine.
- Readiness monitors
- The car's own checks of its emissions systems. An unfinished check means the car has not tested that system yet, so a clean code scan may miss a fault.
- Exhaust recirculation (EGR)
- A system that sends some exhaust back into the engine to reduce emissions. Comparing requested and measured readings can help spot a stuck valve.
- Engine load
- How hard the engine is working, rather than how fast it is turning. Load and turning force (torque) help put other readings in context.
Find out what’s causing yours
Car Assist reads your car through an OBD-II adapter and your iPhone. Its assistant uses the codes, freeze frame and live readings to work through the likely cause with you.