P0400🌫 Emissions / EGRDrive with caution, fix soon

P0400: EGR Flow Malfunction (Meaning, Causes, Fixes)

Reviewed 2026-10-23 · Based on 2 cited source(s)

What P0400 means

The EGR system feeds a small amount of exhaust back into the cylinders. Because exhaust is inert, it lowers peak combustion temperature and cuts oxides of nitrogen (NOx). RepairPal says it is usually no more than 10 percent of the intake air and keeps combustion below about 2500 F, where NOx forms. The powertrain control module (PCM) decides how much EGR to use based on engine load, temperature and other conditions. OBD-Codes.com says P0400 sets if the PCM detects that the exhaust gas entering the cylinders was insufficient or non-existent. RepairPal defines it as the EGR flow monitoring criteria not being met, and says the monitor usually runs during steady freeway and steady city driving. RepairPal says the PCM judges EGR flow in several ways: a temperature rise in the EGR passages, a change in manifold pressure, a drop in the front oxygen sensor signal, EGR valve position sensor movement, knock sensor activity, or a change in the DPFE (differential pressure feedback) sensor reading. It is a generic OBD-II code, though repair steps vary by make and model. The sources differ in a few ways. The names differ slightly (OBD-Codes.com: Exhaust Gas Recirculation Flow Malfunction; RepairPal's heading: Exhaust Gas Recirculation "A" Flow). OBD-Codes.com describes only too little flow and a vacuum-controlled valve, while RepairPal says it can also mean too much flow and lists excessive vacuum to the valve and bad feedback sensors as causes. For symptoms, OBD-Codes.com expects only the check engine light, while RepairPal says some cars ping under load or at higher speeds. RepairPal rates it Moderate; OBD-Codes.com gives no rating.

Severity: can I keep driving?

Drive with caution, fix soon

Get it diagnosed within a reasonable time. RepairPal rates P0400 Moderate, meaning it may not threaten safety or drivability right away. OBD-Codes.com gives no severity rating and says you likely won't notice anything beyond the check engine light, though combustion temperature and NOx emissions go up. If the engine pings or knocks badly under load, or misfires, get it checked sooner.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Plugged or restricted EGR passages, usually from carbon buildup (both sources)
  2. Faulty EGR valve (both sources)
  3. Faulty EGR solenoid, or its wiring or harness (OBD-Codes.com; RepairPal lists the EGR vacuum supply solenoid)
  4. Damaged or disconnected vacuum lines, or a lack of proper vacuum or electrical signal to the EGR valve (both sources)
  5. Excessive vacuum flow to the EGR valve (RepairPal only)
  6. Bad feedback from the MAP sensor, DPFE sensor, EGR valve position sensor or EGR temperature sensor (RepairPal only)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Record the freeze frame data so you know the conditions when the code set, and try to repeat them with a data-streaming scan tool connected (RepairPal).
  2. With the engine running, use a scan tool to open the EGR valve (OBD-Codes.com). If the engine stumbles, the problem was likely intermittent wiring or an intermittent blockage.
  3. If it doesn't stumble, open the valve manually if your design allows it, for example with a vacuum pump. OBD-Codes.com and RepairPal both say that if the engine doesn't stumble and die, the ports are likely plugged and need cleaning. RepairPal adds that a stumble without dying suggests a restriction, and no RPM change suggests the system is plugged.
  4. Check all vacuum lines and hoses for damage, check that the valve gets its specified vacuum, and inspect the solenoid and its harness (both sources). OBD-Codes.com notes most solenoids run on a duty-cycled voltage, so they usually need a scan tool to test.
  5. RepairPal's flow test: at 3000 RPM, opening the EGR valve fully should drop manifold vacuum by at least 3 inches of mercury. If it doesn't, there is a flow or restriction problem.
  6. RepairPal also suggests checking the EGR position sensor by raising and lowering the valve, checking the DPFE reading changes to spec, and confirming the front oxygen sensor reading drops and short-term fuel trim rises when EGR opens.
  7. Don't assume it's the valve. RepairPal says P0400 is often not a problem with the EGR valve itself.

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • The engine pings, knocks or misfires
  • You don't have a scan tool that can command the EGR valve or show feedback sensor data
  • The code returns after cleaning the EGR passages or replacing the valve
  • There are misfire codes along with P0400
  • You need to pass an emissions inspection soon

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0400?

Usually yes for a short time, if the engine runs normally. RepairPal rates it Moderate and both sources say there are often no symptoms beyond the check engine light. Get it diagnosed reasonably soon, and sooner if the engine pings or misfires.

Is P0400 a bad EGR valve?

Not always. Both sources list a faulty valve, but RepairPal says P0400 is often not the valve itself. Carbon-plugged passages, vacuum lines, the EGR solenoid or a feedback sensor are also common causes, so test before buying parts.

What's the difference between P0400 and P0401?

P0401 specifically means insufficient EGR flow was detected. P0400 is the general EGR flow malfunction code: OBD-Codes.com describes it as too little or no flow, while RepairPal says it means the EGR flow monitor's criteria weren't met, which can include too much flow.

How much does it cost to fix P0400?

Neither source gives a repair cost for P0400. It can be carbon cleaning, a vacuum line, a solenoid, a sensor or the valve, so diagnose it first.

Sources

  1. OBD-Codes.com: P0400 Exhaust Gas Recirculation Flow Malfunction
  2. RepairPal: P0400 Exhaust Gas Recirculation "A" Flow (rated Moderate)

Last reviewed 2026-10-23. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.