Emissions control: EGR, DPF, catalytic converter and AdBlueGeneric OBD-II codeUrgency: Garage soon
Code P0400: EGR flow malfunction
P0400 means: exhaust gas recirculation (EGR): flow malfunction.[1][2][3] P0400 is the most general definition in the EGR family: the ECU has detected that the flow of exhaust gas recirculating into the intake is not what it expects. The sources qualify it differently: Autodoc speaks of insufficient or blocked recirculation; Pierburg, of a valve that does not open, so there is no recirculation or it is not detected; and TAT, of a flow that departs from what is expected, whether too low or too high.[1][3][4]
What code P0400 means
P0400 is the most general definition in the EGR family: the ECU has detected that the flow of exhaust gas recirculating into the intake is not what it expects. The sources qualify it differently: Autodoc speaks of insufficient or blocked recirculation; Pierburg, of a valve that does not open, so there is no recirculation or it is not detected; and TAT, of a flow that departs from what is expected, whether too low or too high.[1][3][4]
The EGR sends part of the exhaust gas back into the intake to lower combustion temperature and reduce nitrogen oxides (NOx). According to Samarins, the most common situation is that the flow is lower than expected because soot blocks the system's passages and pipes.[2][5]
It is a generic OBD-II code: the 0 after the P shows it is defined by the SAE standard, so it means the same on any make, and the 4 places it among the auxiliary emission controls, the EGR family. Autodoc cites it as one of the most common exhaust faults, especially on high-mileage diesels.[1][6]
Original definition (SAE J2012): Exhaust Gas Recirculation Flow Malfunction[1][2][3]
Causes of P0400
From most to least likely, according to the sources.
- 1Soot and carbon deposits blocking the EGR valve, its passages or the recirculation pipe: according to Samarins, this is what happens in most cases.[1][2][7]
- 2Seized or faulty EGR valve that does not open when the ECU commands it; on electric valves, also the motor or solenoid that moves it.[1][2][4][7]
- 3Faulty EGR valve position sensor.[1][2][7]
- 4Damaged, blocked or badly connected vacuum hoses or lines, on pneumatically operated EGR valves.[1][2][4]
- 5Electrical fault: loose or corroded connectors, damaged wiring, or problems in the valve's control circuit and power supply.[1][2][4][8]
- 6Restricted exhaust, such as a blocked catalytic converter.[2]
- 7Blocked EGR cooler.[1]
- 8Faulty air mass meter or incorrect ECU software.[1][2]
Symptoms
- Engine warning light on; according to Samarins, sometimes there are no other symptoms.[2][7][8]Engine warning light: faults that cause it
- Rough idle; if the valve sticks open, the engine speed may hunt or the engine may stall.[1][2][4][7]Vibrations: faults that cause it
- Loss of power, especially at low revs.[1][4][7]Less power: faults that cause it
- The engine may go into limp mode (reduced performance).[4]Limp mode: faults that cause it
- Black smoke from the exhaust, with visible soot particles.[1]Black smoke: faults that cause it
- Increased fuel consumption.[1][2]Uses more fuel: faults that cause it
- Combustion knock or pinking (the engine "pings"), because of the higher temperature reached when less gas is recirculated.[1][2][7]
- Higher emissions, with a risk of an unfavourable result at the ITV (Spain's roadworthiness inspection).[1][7]
How to diagnose P0400
In order: from the most likely and cheapest to check to the most expensive.
- 1Start with the simplest and cheapest checks: vacuum hoses (if the EGR valve is pneumatic), and the connectors and wiring of the valve and its sensor. Look for cracks, badly connected pipes, corrosion and damaged wires.[1][2][4][8]
- 2With a diagnostic scan tool, command the EGR valve to open and check that the engine reacts: the idle should drop slightly.[7]
- 3On a pneumatic valve, test with a hand vacuum pump: if it does not hold vacuum, the valve is faulty; if it does not move, check that the vacuum lines are clear and airtight.[4]
- 4Remove the valve and examine it for soot, damage or discoloured areas; also check the intake manifold passages and the EGR cooler, and clean whatever is blocked.[1][2][4][7]
- 5Check the EGR position sensor against the scan tool's reference values before condemning the valve; if you suspect the air mass meter, check its readings too.[1][2][7]
- 6Verify that the exhaust flows freely (especially if you suspect a blocked catalytic converter) and, on turbo engines, that boost pressure control is working.[2][4]
- 7If the valve was seized, find out why it is getting dirty before fitting another: Pierburg recommends checking the injection system and the crankcase oil vapour separator (blow-by).[4]
- 8When you have finished, check for ECU software updates, reset the adaptation values and clear the code. The system needs to complete a driving cycle without errors before it considers the EGR healthy.[1][7]
Can I keep driving with P0400?
Garage soon
The sources do not fully agree. Autodoc Club allows continued driving only for a short period and asks for it to be corrected as soon as possible, because a faulty EGR increases emissions and can end up damaging the engine. Samarins qualifies that it depends on the cause: if it is a minor fault, such as a temperature sensor, and the EGR works, there is no problem; but if it does not work, combustion runs hotter and over time it can damage the engine. Pierburg adds that the engine may go into limp mode, and Diariomotor warns that an EGR left blocked for a long time could affect the injectors, the intake manifold or the particulate filter.[1][2][4][5]
Documented faults related to P0400
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
EGR valve: the most serious faults by engine and car
- Critical severityFrequency: Very frequentBMW B47EGR cooler leaks with risk of engine fireYes: the EGR valve cooler can suffer internal coolant leaks which, mixed with soot and oil, can reach combustion temperatures; BMW has launched several technical campaigns since 2018 to inspect and replace it because of the risk of engine fire.Repair (garage): €500–€1,500
- Critical severityFrequency: Very frequentBMW N57EGR cooler leaks with risk of engine fireYes: like the N47 and B47, the N57 is covered by BMW's service campaign for EGR cooler leaks, which can mix coolant with soot and oil to combustion temperatures. BMW checks free of charge the 6-cylinder engines built between July 2012 and June 2015 because of the fire risk.
- High severityFrequency: Very frequentOM651EGR and particulate filter clogging, and emissions software updatesThe particulate filter and EGR clog up with short journeys, as on any diesel with these systems. In addition, Germany's KBA required software updates on several models with this engine between 2018 and 2021 because of a function that reduced EGR outside a temperature range.Repair (garage): €800–€2,500
- Medium severityFrequency: Very frequentOM642Emissions defeat device (AdBlue/EGR) and software update campaignGermany's KBA found indications of emissions manipulation in cars with the OM642 engine (alongside the OM651): AdBlue dosing and the EGR changed between the test bench and the road. Mercedes-Benz denied it, but in 2018 it recalled hundreds of thousands of diesels in Europe.
Carbon build-up in the intake: the most serious faults by engine and car
- Medium severityFrequency: Very frequent2.0 TSI / TFSICarbon build-up on the intake valvesBecause the fuel is injected directly into the cylinder, no petrol washes over the intake valves, so oil residue and soot build up and, over the years, restrict airflow and affect engine response.
- Medium severityFrequency: Common1.6 HDiLeaks at the injector washers and carbon build-up in the cylinder headIt is common for the copper washers of the 1.6 HDi injectors to stop sealing: the engine blows gases, carbon forms around the injector and soot gets into the oil. It is cheap to repair, but if left it contaminates the oil and is one of the causes of turbo failure.Repair (garage): €110–€310
- Medium severityFrequency: Common1.0 T-GDICarbon build-up on the intake valves and coking of the injectorsThe direct injection on the 1.0 T-GDI does not wash the intake valves with fuel, so over the kilometres carbon builds up on them and on the injectors. It causes loss of power, jerking and higher fuel consumption; it is fixed by cleaning the manifold and valves, without damaging the engine.
- Medium severityFrequency: Common1.6 GDI / T-GDICarbon build-up on the intake valvesBeing pure direct injection, with no port injectors to wash the valves, the 1.6 GDI and T-GDI build up intake carbon with mileage: unstable idle, jerking or loss of power. A design trait of all direct injection, not a manufacturing fault.
Engine sensors: the most serious faults by engine and car
- High severityFrequency: Occasional2.0 DTI and 2.2 DTIVP44 injection pump and air mass meter failureGerman Wikipedia documents recurring failures of the Bosch VP44 injection pump and the air mass meter in the early years of this engine. An owner testimony for this engine describes the pump being replaced at 192,500 km, at a cost the owner calls «prohibitive».
- Medium severityFrequency: Common2.0 TDIClogged particulate filter or faulty pressure sensorThe 2.0 TDI's particulate filter clogs if the car mostly makes short journeys, because it never gets to regenerate; sometimes the fault is just the differential pressure sensor. Cleaning it costs around €200 and replacing it, €800 to €3,000.Repair (garage): €800–€3,000
- Medium severityFrequency: Common1.4 and 1.6 8vCrankshaft position sensor failure (TDC sensor)The crankshaft position sensor (capteur PMH, or TDC sensor) appears repeatedly in the reliability sources for the Clio II, the Kangoo I and the Dacia Logan I with this engine, with hard cold starting or stalling as the main symptoms. It is a cheap part, but the failure can leave you stranded.
- Medium severityFrequency: Common1.4 and 1.6 16vCrankshaft position sensor failureThe crankshaft position sensor (capteur PMH) appears repeatedly in the reliability sources for almost every generation with this engine, with hard cold starting, stalling or the car cutting out unexpectedly. It is a cheap part, but the failure can leave you stranded.
Frequently asked questions
What is the difference between P0400 and codes P0401, P0402, P0403 and P0404?
All five are generic exhaust gas recirculation codes, but each points to something different. P0400 is the most general: the EGR flow is not what is expected, with no further detail. P0401 indicates insufficient flow; P0402, excessive flow; P0403, an EGR circuit malfunction, and P0404, a circuit with out-of-range or performance values. If the scan tool shows P0400 together with one of the others, the more specific one gives the best clue about where to start.[3][4]
Can I drive with code P0400?
Only for a short time and with the intention of repairing it soon. Autodoc Club allows it on that condition; Samarins points out that it depends on the cause and that, if the EGR is not working, the engine runs hotter and may be damaged in the long term; and Pierburg warns that the engine may enter limp mode.[1][2][4]
Is it fixed by cleaning the EGR valve?
Often yes, if the cause is soot: according to Samarins this is what happens in most cases, and Carista recommends inspecting the valve and passages and cleaning or replacing depending on their condition. But an EGR that is not cleaned regularly or that has become blocked several times ends up needing replacement (Diariomotor), and if the valve was seized Pierburg advises also checking the injection and the oil vapour separator to find out why it gets dirty.[2][4][5][7]
Why do diesel EGR valves block up so much?
The recirculating exhaust gas carries soot, so over time the build-up of carbon deposits is almost inevitable. Diariomotor notes that short city trips and low-rev driving speed up the problem, and Autodoc cites P0400 as an especially common fault on high-mileage diesels.[1][5]
Will I pass the ITV with P0400 active?
No: an engine warning light that is on is a serious defect at the ITV (Spain's roadworthiness inspection), and on Euro 5 and Euro 6 cars the inspection also reads the OBD system. In addition, Carista reminds that, after the repair, the system needs to complete its EGR checks without errors before the car can be considered fixed.[1][7][9]
Does P0400 mean the same on all makes?
Yes. It is a generic OBD-II code: those that start with P0 are defined by the SAE standard and have the same definition for all manufacturers. What varies from one engine to another is the type of EGR valve (pneumatic, electric or electromotive) and the most likely cause.[4][6]
Not sure what is wrong with your car? Describe what you notice in the symptom diagnosis or see all fault codes.
Sources
Every fact on this page links to its source. If you find an error, let us know.
- Código OBD2 P0400: causas, síntomas y cómo corregirloAutodoc Club · Technical documentation · consulted 2 October 2026 · in Spanish
- P0400 Exhaust Gas Recirculation Flow MalfunctionSamarins.com · Technical documentation · published 7 July 2021 · consulted 2 October 2026
- P0400: Exhaust Gas Recirculation Flow MalfunctionThe Automotive Technician (TaT) · Technical documentation · consulted 2 October 2026
- OBD-SystemMS Motorservice (Pierburg) · Manufacturer · consulted 2 October 2026
- Síntomas de que la EGR de tu coche diésel está obstruidaDiariomotor · Motoring press · published 1 March 2020 · consulted 2 October 2026 · in Spanish
- On-board diagnosticsWikipedia · Other source · consulted 2 October 2026
- P0400: OBD-II code P0400 flags an EGR flow faultCarista · Technical documentation · consulted 2 October 2026
- Fault Code 16784/P0400/001024Ross-Tech Wiki · Technical documentation · consulted 2 October 2026
- Desmontando mitos: ¿puede pasar mi coche la ITV con el testigo de «avería motor» encendido?Diariomotor · Motoring press · published 8 March 2021 · consulted 2 October 2026 · in Spanish