P0400: EGR A Flow Fault
P0400 means the engine computer found a fault in the flow of exhaust gas recirculation (EGR) system A, without classing it as too little or too much flow. It turns on the check engine light and is not usually urgent, but it can make you fail an emissions test.
You can drive, but get it checked soon
The data does not list limp mode for this code, so the car normally keeps running, with EGR usually switched off. Depending on the direction of the fault you may get pinging under load or a rough idle, and the car will likely fail an emissions test, so have it checked soon.
What it means
The EGR system sends a portion of the exhaust back into the intake to lower combustion temperature and reduce nitrogen oxides (NOx), a regulated pollutant. P0400 is the general flow fault for EGR system A. It is used when the computer does not classify the problem as too little flow (P0401) or too much flow (P0402). On Ford gasoline engines with a stepper-motor valve, the computer opens and closes the valve at steady part load and compares intake manifold pressure with and without EGR. If the difference is too small, it sets P0400, and the light comes on after two trips. GM diesels use the code differently, in air-mass checks when the EGR rate is excessive. The computer then usually switches EGR off.
Likely causes
- EGR passages or valve carboned upVery likely
Carbon deposits from exhaust build up in the passages and on the valve and restrict the flow the computer expects.
- EGR valve stiff or faultyPossible
A valve that sticks or does not move properly cannot open and close as commanded, so the flow check fails.
- Leak in the intake or EGR pipePossible
A leak lets air or exhaust escape or enter where it should not, which changes the pressure readings the test relies on.
- MAP or MAF sensor inaccurateLess likely
The computer judges EGR flow from the manifold pressure (MAP) and air mass (MAF) sensor readings, so a sensor that reads wrong can trigger the code.
What you may notice
- Check engine light on
- Pinging (a light metallic knocking) under load, or a rough idle, depending on the direction of the fault
- Higher NOx emissions and a failed emissions test
What to check, cheapest first
- 1Read the code and any related codes (P0401, P0402, P0403, P0404, P0106) and note freeze-frame data, which shows the conditions when the fault was set. · you can do it
- 2Visually inspect the EGR pipe, vacuum hoses and intake connections for cracks, loose clamps or disconnected parts. · you can do it
- 3Check the EGR valve and its passages for heavy carbon buildup and see whether the valve moves freely. · you can do it
- 4Compare MAP and MAF sensor readings with a scan tool at idle and steady load to see if they look plausible. · mechanic
- 5Command the EGR valve open and closed with a scan tool and watch the intake manifold pressure change. · mechanic
- 6After repair, clear the code and complete a drive cycle to confirm the fault does not return. · mechanic
Related codes
Frequently asked questions
Can I drive with P0400?+
Usually yes for a short time, since limp mode is not typical for this code. Get it checked soon, because you may notice pinging or rough idle and the car may fail emissions.
Can a loose gas cap cause P0400?+
Not according to the reference data. The listed causes are carbon buildup, a faulty valve, a leak in the intake or EGR pipe, and inaccurate MAP or MAF sensors.
Will P0400 pass an emissions test?+
Unlikely. The code is emissions-relevant and the check engine light is on, and EGR problems raise NOx, so expect a fail until it is repaired.
Can I fix it myself?+
Often yes. The repair is rated medium difficulty, DIY is possible, and the estimated time is 1 to 3 hours.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10