# P0400: Exhaust Gas Recirculation "A" Flow **Verdict:** Safe to drive Emissions - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $150-$800 - **Urgency:** Keep driving - **Most likely:** Carbon-clogged or stuck EGR valve - **DIY?:** Steps 1-2 ## Quick answer P0400 means the engine computer has found a general problem with exhaust gas recirculation (EGR) flow, the system that routes a small amount of exhaust back into the engine to cut emissions. It does not say whether there is too little or too much flow, only that flow is not what the computer expected. Carbon buildup, a stuck EGR valve, or a vacuum or sensor problem are the usual causes. ## What does P0400 mean? The exhaust gas recirculation (EGR) system feeds a small, metered amount of exhaust back into the intake. Exhaust has already burned, so it soaks up heat and lowers peak combustion temperature, which cuts nitrogen oxide (NOx) emissions and helps prevent pinging. The EGR valve opens mostly at steady cruise and light acceleration, and stays closed at idle and full throttle, when the engine needs as much fresh air as it can get. P0400 is the general EGR flow code. The engine computer runs an EGR test, often during deceleration or steady cruise, by opening the valve and watching for an expected change in intake manifold pressure, a flow sensor reading, or the fuel mixture. If the result is out of range, it stores a pending code and usually turns on the light if the fault repeats on a second drive cycle. The "A" in the title simply means the first or only EGR system. Many cars set the more specific P0401 or P0402 instead, and many newer engines have no external EGR valve at all. ## What are the symptoms of P0400? - Check engine light on - Pinging or knocking when accelerating (too little flow) - Rough idle or stalling (too much flow) - Failed emissions test, often for high NOx ## What causes P0400? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Carbon buildup in the EGR valve or passages** (most common). Exhaust carries fine soot that sticks to the valve, its seat, and the narrow passages that feed exhaust into the intake. Over many miles it hardens into carbon that restricts or blocks flow. It builds faster on high-mileage engines, engines that burn oil, and cars driven mostly on short trips that never fully warm up. 2. **Failed EGR valve** (common). The EGR valve itself can stick open or closed, or its electric motor, position sensor, or rubber diaphragm can fail. Electronic valves often set extra position or circuit codes alongside P0400. A valve stuck open tends to cause a rough idle or stalling, while one stuck closed more often causes pinging under load. 3. **Vacuum hose or EGR solenoid problem (vacuum-operated systems)** (less common). Older systems open the EGR valve with engine vacuum, switched on and off by a control solenoid. A cracked, collapsed, or misrouted hose, or a solenoid that no longer switches, means the valve never gets the signal to open. This mostly affects vehicles from the 1990s and 2000s, since newer EGR valves are usually fully electronic. 4. **Faulty EGR feedback sensor** (less common). Many systems measure EGR flow or valve position with a separate sensor, such as the DPFE pressure sensor on many older Fords or a position sensor on top of the valve. If that sensor fails, or its small rubber hoses crack or fall off, it reports the wrong flow and the computer flags a valve fault that is not there. ## How do you diagnose P0400? 1. Read all codes and freeze-frame data. Look for P0401, P0402, or P0403 that narrow the problem. 2. Inspect vacuum hoses and connectors at the EGR valve and any feedback sensor. 3. Command the EGR valve open with a scan tool at idle. The engine should stumble noticeably if exhaust is flowing. 4. Remove the valve and inspect it and the ports for carbon if flow is weak. ## How do you fix P0400, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Clean EGR valve and passages | $150-$450 | Labor-heavy on engines where passages run through the intake manifold. | | Replace EGR valve | $250-$750 | Electronic valves cost more than older vacuum-operated ones. | | Replace EGR feedback sensor or vacuum solenoid | $100-$300 | Fixes a faulty measurement or control part rather than the valve itself. | ## Is it safe to drive with P0400? > **Safe to drive.** It is usually safe to drive for a while. Get it checked if you hear pinging under acceleration or the engine idles rough or stalls, since those can lead to drivability problems or engine damage over time. ## Will P0400 fail an emissions test? With P0400 stored and the check engine light on, a car will fail an OBD-based emissions test in most states. Clearing the code does not help, because it resets the EGR and other readiness monitors to not ready, and states limit how many incomplete monitors they accept. On tailpipe-test programs, too little EGR flow usually shows up as high NOx. ## After the repair: clearing P0400 After the repair, clear the code with a scan tool. The EGR monitor then has to rerun, which usually takes a few days of mixed driving that includes steady cruising and long, gentle slowdowns from highway speed with your foot off the pedal. The fix worked when the EGR monitor shows ready and no pending P0400, P0401, or P0402 comes back. ## How to prevent P0400 - Mix in regular highway drives, since constant short trips let soot build up in EGR passages faster. - Fix oil burning and PCV problems early, because oil vapor mixes with EGR soot and bakes into hard carbon. - Repair misfires and rich running promptly, since they send extra soot into the exhaust that feeds the EGR system. - Replace cracked vacuum hoses and sensor hoses when you see them, especially on older vacuum-operated EGR systems. - Ask for an EGR passage check during intake or tune-up work on engines known for carbon buildup. ## Codes often seen with P0400 - [P0401](/codes/p0401): Exhaust Gas Recirculation "A" Flow Insufficient Detected - [P0402](/codes/p0402): Exhaust Gas Recirculation "A" Flow Excessive Detected - [P0403](/codes/p0403): Exhaust Gas Recirculation "A" Control Circuit - [P0300](/codes/p0300): Random/Multiple Cylinder Misfire Detected ## Frequently asked questions ### Can I drive with a P0400 code? Yes, most cars drive normally with P0400 for a while as you plan a repair. Fix it sooner if you hear pinging under acceleration, because long-term knock can damage pistons, or if the engine stalls at stops. You will also likely fail an emissions test until the EGR system works again. ### What is the difference between P0400 and P0401? P0400 is a general EGR flow fault, while P0401 specifically means too little flow and P0402 means too much. Which code you get depends on how the maker wrote its software. Some cars only ever set P0400, so a shop still has to test whether flow is too low or too high. ### Will cleaning the EGR valve fix P0400? Cleaning often fixes P0400 when carbon is blocking the valve or its passages. If the valve's motor or diaphragm has failed, or the feedback sensor is reading wrong, cleaning will not help and that part needs replacing. A shop can usually tell which by watching EGR data on a scan tool. ### How much does it cost to fix P0400? Most P0400 repairs cost $150 to $800 at an independent shop. A feedback sensor or vacuum solenoid is usually $100 to $300, cleaning runs $150 to $450, and a new electronic EGR valve costs $250 to $750. Engines whose EGR passages run through the intake manifold sit at the top of that range. ### Will P0400 make me fail an emissions test? Yes, a car with P0400 and the check engine light on will fail an OBD-based emissions test in most states. Clearing the code right before the test does not work, because the EGR readiness monitor will show not ready. Repair it first, then drive normally until the monitors complete. ### Can I fix P0400 myself? You can often handle the first steps yourself, such as checking vacuum hoses, inspecting connectors, and removing an easy-to-reach valve to look for carbon. Cleaning the valve is a reasonable DIY job on many older engines. Testing feedback sensors or cleaning passages inside the intake manifold is usually a shop job. ### What are the symptoms of a bad EGR valve? The most common signs are a check engine light, pinging or knocking when accelerating, and a rough idle or stalling at stops. Too little flow tends to cause pinging and high NOx, while too much flow causes rough idle and hesitation. Many cars with P0400 show no drivability change at all. ### Can P0400 damage my engine? It can over time, but usually not quickly. Without enough EGR, combustion runs hotter and some engines ping under load, and prolonged knock can damage pistons and head gaskets. A valve stuck open is more of a stalling and traffic safety risk than an engine damage risk, so repair it either way. ### Can I delete or block off the EGR valve? No, removing, blocking, or disabling the EGR system on a road vehicle is emissions tampering under the federal Clean Air Act. It also keeps the check engine light on, raises NOx emissions, and can cause pinging. The engine computer is calibrated to expect EGR, so a working system is the only lasting fix. ### Why does P0400 come back after cleaning the EGR valve? It usually returns because the passages feeding the valve were left blocked, or because a feedback sensor or vacuum part was the real fault. On some engines the clogged channels run inside the intake manifold and need it removed to clean. An engine that burns oil can also rebuild carbon quickly. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other emissions codes - [P0401 Exhaust Gas Recirculation "A" Flow Insufficient Detected](https://diagxcar.com/codes/p0401) - [P0402 Exhaust Gas Recirculation "A" Flow Excessive Detected](https://diagxcar.com/codes/p0402) - [P0403 Exhaust Gas Recirculation "A" Control Circuit](https://diagxcar.com/codes/p0403) - [P0404 Exhaust Gas Recirculation "A" Control Circuit Range/Performance](https://diagxcar.com/codes/p0404) - [P0405 Exhaust Gas Recirculation Sensor "A" Circuit Low](https://diagxcar.com/codes/p0405) - [P0406 Exhaust Gas Recirculation Sensor "A" Circuit High](https://diagxcar.com/codes/p0406) - [P0410 Secondary Air Injection System "A"](https://diagxcar.com/codes/p0410) - [P0411 Secondary Air Injection System "A" Incorrect Flow Detected](https://diagxcar.com/codes/p0411) - [P0420 Catalyst System Efficiency Below Threshold (Bank 1)](https://diagxcar.com/codes/p0420) - [P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)](https://diagxcar.com/codes/p0421) ## Symptoms - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) - [Why does my car stall while driving?](https://diagxcar.com/symptoms/car-stalls-while-driving) ## Repair costs - [EGR valve replacement cost](https://diagxcar.com/cost/egr-valve-replacement) ## Terms explained - [What is EGR valve?](https://diagxcar.com/glossary/egr-valve) --- Source page: https://diagxcar.com/codes/p0400 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.