P0400: EGR A Flow
A generic powertrain (engine and transmission) trouble code, same meaning on every brand.
Yes - expect slightly rough running and a failed emissions test.
Symptoms
- Rough idle or stumble at light throttle
- Pinging under load
- Emissions test failure
Likely causes, most common first
- EGR passages blocked with carbon
- EGR valve stuck closed
- Vacuum or electrical control fault
- Differential pressure sensor or its hoses blocked
Read P0400 yourself before paying for diagnosis
A basic Bluetooth OBD-II scanner ($20-40) plugs into the port under the dashboard and shows this code with live sensor data on your phone - useful both for confirming the fix and for talking to a shop from an informed position.
What is going on and how it gets diagnosed
P0400 says exhaust gas recirculation flow is not what the computer expects. Recirculating a measured amount of inert exhaust gas lowers combustion temperature, which cuts nitrogen oxide emissions and reduces the tendency to knock.
Carbon is almost always the answer. The passages that carry exhaust gas back into the intake collect soot over years and eventually close up, and on many engines cleaning those passages - not replacing the valve - is the actual repair. Fitting a new valve to blocked passages produces the same code within weeks.
Repair options and cost
Typical US independent-shop ranges, parts and labor. Dealers run higher; regional labor rates vary.
Cars that report P0400 most often
Based on mentions of this code in NHTSA owner complaints.
Frequently asked questions
Can I drive with P0400?
Yes - expect slightly rough running and a failed emissions test.
How much does it cost to fix P0400?
Typical US repair costs range from $120 for the simplest fix (egr passage cleaning) to $700 in the worst case. The right number depends on which cause your car has - see the causes list.
Related codes
The codes closest to P0400. They often appear together, and telling them apart is usually what narrows the diagnosis.
Definition per SAE J2012 standard nomenclature. Symptoms, causes and costs are editorial guidance - always confirm with a diagnosis of your specific vehicle.