P0400 Exhaust Gas Recirculation Flow
P0400 is a generic OBD2 powertrain code. It means “Exhaust Gas Recirculation Flow” — the same on every make and model, because the definition comes from the SAE J2012 standard rather than from the carmaker.
Ask the AI about P0400 on your car- System
- Powertrain (P)
- Code type
- Generic (SAE)
- Range
- P04xx
- Urgency
- Low
What P0400 means
Exhaust gas recirculation (EGR) feeds a metered amount of exhaust back into the intake to cool combustion and cut NOx. The computer opened the EGR valve and saw less effect than expected (P0401), or found an EGR flow fault in general (P0400).
Symptoms
- Often nothing but the lamp
- Pinging or knocking under load, especially when hot
- A failed emissions test (high NOx)
Most likely causes
- Carbon clogging the EGR passages or the valve — the most common by far
- A failed EGR valve or its control solenoid
- A faulty EGR position or differential-pressure (DPFE) sensor
- A cracked vacuum hose on vacuum-operated valves
What to check first
- Remove and inspect the valve and the passages — cleaning often fixes it
- Check the vacuum hoses and the valve’s connector
- Test the EGR feedback sensor before replacing the valve
Can I keep driving with P0400?
Safe to drive — fix it when convenient
Yes. Listen for knocking under load — sustained knock can damage pistons — and fix it before the next emissions test.
How P0400 breaks down
Powertrain. Engine, transmission, fuel and emissions — the codes behind the check engine lamp.
Generic code: defined by SAE, the same on every make.
Auxiliary emission controls.
The specific fault within this range.
Where to look: Auxiliary emission controls
The systems that clean up exhaust and fuel vapor: exhaust gas recirculation (EGR), secondary air injection, catalytic converter monitoring, the evaporative emissions (EVAP) system with its purge and vent valves — plus exhaust temperature sensors, the particulate filter and some cooling-fan circuits.
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Frequently asked
What does P0400 mean?
P0400 is a generic OBD2 powertrain code. It means “Exhaust Gas Recirculation Flow” — the same on every make and model, because the definition comes from the SAE J2012 standard rather than from the carmaker. Exhaust gas recirculation (EGR) feeds a metered amount of exhaust back into the intake to cool combustion and cut NOx. The computer opened the EGR valve and saw less effect than expected (P0401), or found an EGR flow fault in general (P0400).
Can I drive with P0400?
Yes. Listen for knocking under load — sustained knock can damage pistons — and fix it before the next emissions test.
What causes P0400?
Carbon clogging the EGR passages or the valve — the most common by far; A failed EGR valve or its control solenoid; A faulty EGR position or differential-pressure (DPFE) sensor; A cracked vacuum hose on vacuum-operated valves.
How do I clear P0400?
Fix the cause first, then clear it with any OBD2 scanner or app. Once the fault is gone the car also clears it on its own: the check engine lamp goes off after the self-test passes on three drive cycles in a row, and the stored code ages out after about 40 warm-ups. Clearing without a repair only hides it for a while — and resets the readiness monitors an emissions inspection looks at.
Is P0400 the same on every car?
Yes. P0400 is a generic SAE J2012 code, so its definition is identical on every make and model. What differs is the cause behind it and the repair procedure, which depend on the engine — that’s why the assistant on this page asks for your car.
Guides
Definitions: the generic SAE J2012 codes from Brimly’s offline dictionary (data version 2026.09). Causes and advice are general guidance for all makes — the service information for your car has the last word.