Emission controls · Powertrain · generic (SAE-defined)
P0400: Exhaust Gas Recirculation Flow
P0400 means the engine computer has detected a general problem with exhaust gas recirculation (EGR) flow, without specifying whether flow is too high or too low.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0400 sets
The powertrain control module (PCM) commands the exhaust gas recirculation (EGR) valve open under certain cruise conditions and watches for an expected response, such as a change in intake manifold pressure, EGR position feedback, or a temperature rise at an EGR sensor. If the measured response does not match what was commanded over a test cycle, the PCM sets P0400. The test method and limits are manufacturer-specific, so factory service information is the reference.
Common causes of P0400
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Carbon-clogged EGR passages or intake ports
- EGR valve stuck closed or stuck open
- Faulty EGR valve position sensor or differential pressure feedback sensor
- Cracked, collapsed, or disconnected EGR vacuum hoses
- Failed EGR vacuum solenoid or electrical fault in its circuit
- Clogged or leaking EGR cooler (common on diesels)
P0400 symptoms
- Check engine light on
- Engine pinging or knocking under acceleration
- Rough idle or stalling if EGR is stuck open
- Possible failed emissions test due to elevated oxides of nitrogen (NOx)
- Often no noticeable drivability change
How P0400 is diagnosed
- Scan for all codes and record freeze frame data; address any related EGR circuit or sensor codes first
- Visually inspect EGR vacuum lines, connectors, and the valve for damage or carbon buildup
- Command the EGR valve open with a bidirectional scan tool (or apply vacuum on vacuum-type valves) at idle; engine should stumble noticeably
- Compare EGR position or feedback sensor readings in live data to the commanded value
- Remove the valve if needed and inspect the valve and intake-side passages for carbon blockage
- Test the control solenoid and wiring for proper resistance, power, and ground
Typical fixes for P0400
- Clean carbon from EGR passages, ports, and the valve
- Replace a stuck or failed EGR valve
- Replace the EGR feedback or differential pressure sensor
- Repair or replace vacuum hoses and the control solenoid
- Repair damaged wiring or connectors in the EGR circuit
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Manufacturer notes
Definitions can vary by manufacturer; check factory service information for the exact monitoring strategy on your engine.
Related codes & warning lights
P0400 FAQ
What does code P0400 mean?
P0400 means the engine computer has detected a general problem with exhaust gas recirculation (EGR) flow, without specifying whether flow is too high or too low. The standard definition is "Exhaust Gas Recirculation Flow."
Can I drive with a P0400 code?
Usually yes for short-term driving, but have it diagnosed within days. A stuck EGR system can cause engine knock (pinging) under load or a rough idle and stalling, and prolonged knock can damage pistons.
Is P0400 the same as P0401?
No. P0400 is a general EGR flow fault, while P0401 specifically reports insufficient flow. Many vehicles set the more specific code instead, but the diagnostic path is similar.
Can I just block off the EGR valve?
No. Disabling the EGR system is illegal for road vehicles in the US, will keep the code active, and can cause engine knock and higher combustion temperatures.
Keep researching
More emission controls codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
- California Air Resources Board — On-Board Diagnostic (OBD) Program
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.