What P0400 means
P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 4, places it in the auxiliary emission controls subsystem; the last two digits identify the specific fault.
The EGR system feeds a measured amount of exhaust back into the intake to lower combustion temperature and reduce oxides of nitrogen. The engine computer opens the EGR valve under specific conditions and checks the result, either from a valve position sensor or from the change in manifold pressure.
What “Flow” means here
The module ran its flow test on the exhaust gas recirculation (EGR) system and the measured result did not match what it expected. This is a performance result, so blocked passages, stuck valves and leaks are as likely as electrical faults.
What you may notice
Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0400:
- Pinging or knock under load if flow is low
- Rough idle or stalling if the valve sticks open
- Hesitation
Where to start diagnosing P0400
- Inspect the EGR valve and passages for carbon build-up, a very common cause of flow faults.
- Check vacuum supply on vacuum-operated valves, or the connector and resistance on electric ones.
- Command the valve open with a capable scan tool at idle; the engine should stumble noticeably if exhaust is flowing.
- On diesels, check the EGR cooler for blockage.
Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.
Live data to watch
These standard OBD2 values relate to P0400. Support varies by vehicle; a value your car does not report will simply be missing.
| Value | What to look for |
|---|---|
| Commanded EGR (PID 0x2C) | Desired opening. |
| EGR error (PID 0x2D) | Difference between commanded and actual. |
| Intake manifold absolute pressure (PID 0x0B) | Should change when EGR opens. |
Related codes
Codes from the same exhaust gas recirculation (EGR) system family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.
| Code | Definition |
|---|---|
| P0401 | Exhaust Gas Recirculation (EGR) Flow Insufficient |
| P0402 | Exhaust Gas Recirculation "A" Flow Excessive Detected |
| P0403 | Exhaust Gas Recirculation "A" Control Circuit |
| P0404 | Exhaust Gas Recirculation "A" Control Circuit Range/Performance |
| P0489 | Exhaust Gas Recirculation "A" Control Circuit Low |
| P0490 | Exhaust Gas Recirculation "A" Control Circuit High |
Further reading
Frequently Asked Questions
What does P0400 mean?
P0400 is the generic SAE J2012 code for "Exhaust Gas Recirculation "A" Flow". In plain terms, it means the engine computer's exhaust gas recirculation (EGR) system monitor has reported "Exhaust Gas Recirculation "A" Flow".
Can I keep driving with P0400?
Usually drivable. A valve stuck open can cause stalling at idle; a stuck-closed valve can allow knock under load.
What should I check first for P0400?
Inspect the EGR valve and passages for carbon build-up, a very common cause of flow faults.
Definition source: SAE J2012 / ISO 15031-6 generic DTC definitions. This is the generic meaning that applies across makes; a manufacturer can define a code more narrowly or add its own sub-codes, so confirm against the service information for your vehicle before replacing parts. Written by the OBDAssistant engineering team — see our editorial policy.