OBD-II · P0401
P0401 Code: Restricted EGR Flow — Clean Before You Replace
Insufficient exhaust gas recirculation (EGR) flow
The short version
Drivable, but you may notice light knock under load and higher NOx output, and the car will fail an emissions test.
- Severity: Moderate
- Most common cause: Carbon buildup in the EGR passage or manifold port
- 5 diagnosis steps in order
- 4 symptoms tied to this code
Darker bars are the more common causes; bar length is the top expected repair cost. Start with the dark, cheap causes before the expensive ones.
Can I keep driving?
Drivable, but you may notice light knock under load and higher NOx output, and the car will fail an emissions test.
Symptoms
- Pinging or metallic knock under load
- Rough idle on some models
- Emissions failure with high NOx
- Steady check engine light that returns after clearing
Causes & repair cost
Carbon buildup in the EGR passage or manifold port
Most common$80 – $350
Stuck or failed EGR valve
Most common$150 – $500
Damaged vacuum hose or regulator (older systems)
Possible$30 – $150
Faulty DPFE pressure sensor
Possible$70 – $250
Clogged EGR cooler on diesel engines
Rare$500 – $1,600
Shop cleaning runs $80–$350 and a new valve $150–$500. Cleaning resolves most cases on cars used for short trips.
Estimate the repair for your carDiagnosis steps in order
- Command the EGR valve open at idle with a scan tool — the engine should stumble or stall
- Remove the valve and probe the passage — blockage is visible
- Clean the passage with carbon cleaner and a brush, then retest
- Inspect vacuum hoses and the DPFE sensor where fitted
- Clear the code and complete a full drive cycle to confirm
FAQ
Can I clean the EGR valve myself?
On many engines yes; two bolts, a new gasket, and carbon cleaner suffice — just keep cleaner away from the valve's electrical parts.
Is deleting the EGR a valid fix?
No; deletion is illegal for emissions, raises combustion temperature and NOx, and can damage valves.
Why does the blockage keep coming back?
Short trips, poor fuel, and oil passing through the PCV accelerate carbon buildup; one longer weekly drive helps.
Does the code hurt fuel economy?
The difference is small, but accompanying knock can make the ECM retard timing, slightly raising consumption.
Related codes
All explained codes
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