P0400 Exhaust gas recirculation flow is not what the computer expects

EGR flow malfunction

Monitor

Most bulletins describe a lit check engine light and sometimes rough idle, with no safety warning, so it can usually be driven while you book a check, but it will fail an emissions test.

It gets more serious if:

  • Inspect soon: Inspect soon if the engine idles rough, hard starts, loses power or drops into limp mode, which owners report when an EGR valve fails.

Source: how we rate severity

P0400 in short

P0400 means the computer sees the wrong amount of exhaust gas being recirculated into the intake. Carbon in the EGR valve or passages is the usual cause, then vacuum or wiring faults. Some Subaru and Mitsubishi models need a computer update instead. You can usually drive on, but expect rough idle.

An EGR system sends a measured share of exhaust back into the engine to lower combustion temperature and reduce nitrogen oxide. The computer checks manifold pressure to see whether that flow really happens. When the flow does not match what it expects for engine speed and load, it stores P0400.

TroubleCodes.net and the Delaware DMV list both give the same generic title: EGR flow malfunction. The code says the flow is wrong, not whether it is too little or too much; P0401 and P0402 cover those cases.

Several carmakers have found software behind it. Subaru says the code can set when the EGR test runs below freezing, and Mitsubishi reprograms the computer on the 2019-2023 Outlander Sport. On the Hyundai Tucson Hybrid, owners report an electrical short in the EGR valve and corroded connectors.

Most likely causes

  1. EGR valve or passages clogged with carbon, or a valve that sticks

    Common general cause
    Why it ranks here
    TroubleCodes.net lists EGR valve malfunction, clogged passages and carbon build-up. Nissan also has a bulletin that links P0400 with rough idle on 1998-2000 KA24 Frontier and Xterra.
    What would change this
    Rises if the valve sticks or the passages are blocked when inspected. Falls if the valve moves freely and the passages are clear.

  2. Vacuum leak, solenoid or wiring fault on the EGR control

    Common general cause
    Why it ranks here
    TroubleCodes.net lists vacuum or hose leaks, wiring damage and solenoid failure. Which one depends on whether the system is vacuum-controlled or electronic.
    What would change this
    Rises if a hose is cracked or off, or the connector is damaged. Falls if the vacuum and wiring check out.
    How to test the vacuum hoses and intake leaks

  3. MAP or DPFE sensor fault, or an exhaust restriction

    Possible — needs testing
    Why it ranks here
    TroubleCodes.net lists defective MAP or DPFE sensors, exhaust leaks and clogged converters, because the computer judges EGR flow from those readings.
    What would change this
    Rises if sensor readings look wrong at idle or the exhaust is restricted. Falls if sensor data looks normal.

  4. Engine computer software that needs an update (some Subaru and Mitsubishi models)

    Supported by vehicle data
    Why it ranks here
    Subaru bulletins 11-197-20 and 11-200-20R reprogram the ECM, and the first says the code can set when the EGR test runs below freezing. Mitsubishi TSB-23-13-004 reprograms the ECU on the 2019-2023 Outlander Sport for P0401/P0400.
    What would change this
    Rises on a covered model where the code appears in cold weather and the ECM is not updated. Falls on other makes or if already updated.
    How to test the engine control module (ecm / pcm)

  5. EGR valve electrical fault or corroded connector (Hyundai Tucson Hybrid, owner reports)

    Possible — needs testing
    Why it ranks here
    Owners of 2022 and 2023 Tucson Hybrids report P0400-series codes with limp mode, an internal short in the EGR valve and corrosion in the connector and harness. These are owner reports, not a bulletin, so this ranks low.
    What would change this
    Rises if the connector shows corrosion or the valve circuit reads shorted. Falls if the circuit and valve test normal.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read all stored codes and freeze-frame data with a scan tool, and note the outside temperature and whether other EGR codes (P0401, P0402) are stored.

    Low riskTool: OBD-II scan tool

    • IfThe code appears only in cold weather on a Subaru or Mitsubishi

      It meansA computer update may apply.

      Go to step 2

    • IfThe code is steady or comes with rough idle

      It meansInspect the EGR system.

      Go to step 3

  2. Ask a dealer whether your VIN has an ECM update for P0400. Subaru and Mitsubishi have issued bulletins.

    Low risk

  3. With the engine off and cool, look over the EGR vacuum lines for cracks or loose ends and the EGR connector and wiring for corrosion or damage.

    Low risk

  4. Have a shop check that the EGR valve opens and closes on command, that its passages are clear of carbon, and that the MAP or DPFE readings respond.

    Best left to a shopTool: scan tool, vacuum pump

    Warning:The EGR valve and its tubes sit next to the exhaust manifold and can cause burns when hot. Let the engine cool, and have a shop do it.

Symptoms

  • Check engine light on, often with no other change
  • Rough idle
  • Hard starting, especially in cold weather
  • Loss of power or limp mode Gets more serious: stop and have it checked.
  • Poorer fuel economy or a failed emissions test

How to fix

  1. With the engine off and cool, look over visible vacuum lines near the EGR valve for cracks or a hose that has come off.

  2. Ask a dealer whether your VIN has an open ECM update for P0400, especially on Subaru and Mitsubishi.

Parts and cost context

  • Vacuum hoses and intake leaks:A cracked vacuum hose is the cheapest fix on vacuum-controlled systems. Do not replace the EGR valve until you know whether it sticks or is clogged. Labour varies; get a quote.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Subaru

  • 2015–2020 · Crosstrek

    Supported by vehicle data

    Subaru bulletins 11-197-20 and 11-200-20R reprogram the ECM for P0400, which can set below freezing. Whether they are still current is not verified.

  • 2016–2020 · Forester

    Supported by vehicle data

    The same Subaru ECM updates cover the Forester. Whether they are still current is not verified.

  • 11-197-20Status unverified

    ECM reprogramming for P0400 EGR A flow

    2015–2018crosstrek, forester, impreza · issued · NHTSA 10171917

    Reprograms the ECM because the P0400 test can set when ambient or intake air temperature is below freezing.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • 11-200-20RStatus unverified

    ECM reprogramming for cold hard starting, P0400 and high-altitude surging

    2015–2020crosstrek, forester, impreza, legacy, outback · issued · NHTSA 10175989

    Reprograms the ECM for hard starting in sub-freezing temperatures, P0400 and surging at altitude.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Mitsubishi

  • 2019–2023 · Outlander Sport

    Supported by vehicle data

    Mitsubishi TSB-23-13-004 reprograms the ECU when P0401/P0400 sets. Whether it is still current is not verified.

  • TSB-23-13-004Status unverified

    ECU reprogramming for P0401/P0400

    2019–2023outlander-sport · issued · NHTSA 10241921

    Reprograms the ECU with countermeasure software when the check engine light shows P0401 or P0400.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Nissan

  • 1998–2000 · Frontier

    Supported by vehicle data

    Nissan bulletin 05040A links P0400 and P0402 with rough idle on KA24 engines. Whether it is still current is not verified.

  • 05040AStatus unverified

    KA24 engine: MIL with P0400, P0402 and rough idle

    1998–2000frontier, xterra · issued · NHTSA 10015857

    Update for 1998-2000 Frontier and Xterra KA24 engines with P0400, P0402, rough idle and difficulty passing the NOx test. The summary does not give the repair.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Hyundai

  • 2022–2023 · Tucson

    Possible — needs testing

    Owners of 2022-2023 Tucson Hybrids report P0400-series codes with limp mode, an internal short in the EGR valve and a corroded connector. These are complaints, not a verified repair.

  • P0401Exhaust gas recirculation (EGR) flow insufficient detected

    Same system Supported by vehicle data

    P0401 is EGR flow insufficient. Nissan, Ford and Mitsubishi bulletins list P0400 together with P0401 and P0402, so the same causes often apply.

  • P0402EGR flow higher than the computer expects

    Same system Supported by vehicle data

    P0402 is EGR flow excessive, often stored with P0400 in carmaker bulletins.

All Turbocharging, fuel pressure and EGR codes

P0400 questions

Is P0400 the same as P0401?

No. P0400 means the EGR flow is wrong in general. P0401 means flow is too low and P0402 too high.

Will a software update fix P0400?

On some Subaru and Mitsubishi models, yes. Ask a dealer if your VIN is covered before replacing the EGR valve.

Sources and review

  1. TroubleCodes.net DTC pages (P0342, P0385, P0394, P0400, P0431, P0447; P0413, P0414, P0416, P0417 read but not used) — TroubleCodes.net
  2. Subaru 11-197-20: ECM reprogramming for P0400 EGR A flow (2015-2018 Crosstrek, Forester, Impreza) — Subaru / NHTSA ,
  3. Nissan 05040A: 1998-2000 Frontier and Xterra with KA24 engine: MIL with P0400, P0402, rough idle — Nissan / NHTSA ,
  4. NHTSA complaint ODI 11753488: 2023 Hyundai Tucson Hybrid — NHTSA Office of Defects Investigation (consumer complaint)
  5. Subaru 11-200-20R: ECM reprogramming for hard starting in sub-freezing temperatures, P0400 and high-altitude surging — Subaru / NHTSA ,
  6. Mitsubishi TSB-23-13-004: Reprogram the ECU when the check engine light shows P0401/P0400 (2019-2023 Outlander Sport) — Mitsubishi / NHTSA ,
  7. NHTSA complaint ODI 11754790: 2022 Hyundai Tucson Hybrid — NHTSA Office of Defects Investigation (consumer complaint)
  8. Ford 14742: MIL on with EGR-related DTCs P0400, P0401, P0402 and others (2002 Explorer and Mountaineer) — Ford / NHTSA ,
  9. On-Board Diagnostic Trouble Codes (generic code title list) — Delaware Division of Motor Vehicles
Last reviewed
Written by
Saurabh K.
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