P0400 – Exhaust gas recirculation (EGR) system -flow malfunction

By Reinier (Contact Me)
Last Updated 2016-07-29
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P0400 Mean?
2. What are the common causes of code P0400?
3. What are the symptoms of code P0400?
4. How do you troubleshoot code P0400?
5. Codes Related to P0400
6. Get Help with P0400

What Does Code P0400 Mean?

OBD II fault code P0400 is defined as “Exhaust Gas Recirculation Flow Malfunction”, and is set when the PCM (Powertrain Control Module) detects a problem with the amount of exhaust gas being rerouted to the inlet tract. Such a problem will cause a trouble code to be stored, and a warning light to be illuminated. On some applications, a code will be stored only after several failure cycles, while on others a code will be stored on the first failure.

The function of the EGR (Exhaust Gas Recirculation) system on both gasoline and diesel engines is to redirect a percentage of the exhaust gas into the inlet tract to be included in the air/fuel mixture. Since exhaust gas is relatively oxygen poor, the recirculated exhaust gas dilutes the air/fuel mixture, thereby reducing combustion temperatures to below 1 500⁰C (2 800⁰F), which is the temperature at which nitrous oxide forms. Nitrous oxide is a combination of oxygen and nitrogen, and the chief component in smog caused by car exhaust emissions.

Design details of EGR systems vary rather widely, but the PCM generally calculates the actual amount of exhaust gas being recirculated by monitoring changes in the manifold pressure as the gas is being introduced, or prevented from entering the manifold. When the changes in manifold pressure do not coincide with the signal the PCM expects to receive under any given engine load/speed, a code will be stored.

Note that there are fundamental differences between how EGR systems work on gasoline (spark ignited) engines, and diesel (compression ignited) engines. Below are some details on the specifics;

Spark ignited engines:

On these applications the amount of recirculated exhaust gas is limited to about 10% (or slightly more on some applications), since excessive amount of gas interferes with the flame front (combustion process) that in turn causes misfires and poor combustion. Therefore, the EGR system is deactivated at idling and during high engine loads to ensure optimum combustion, by not diluting the charge (air/fuel) mixture density. An added advantage of EGR is the cooling effect it has on the exhaust valves, extending the service life of these components considerably.

Note that many of the adverse affects on combustion by recirculating exhaust gas can be countered effectively by the PCM by means of advancing the ignition timing.

Compression ignited engines:

Unlike spark ignited engines, diesels always run with excess air. Moreover, diesel combustion does not depend on smooth flame propagation during combustion to the same extent that gasoline engines do, which means that much larger percentages of exhaust gas can be recirculated without adverse affects. Diesel engines can comfortably cope with up to 50% of the exhaust being recirculated at idle, since on these engines, there is always a large excess of air. On many diesel applications, the hot, incoming exhaust gas is passed through a heat exchanger to cool it down before being passed into the engine via a metering valve.

However, large amounts of exhaust gas translate into large amounts of particulate matter being introduced into the engine. Particulate matter in diesel exhaust consists mainly of sooty carbon, which can increase engine wear, and particularly when the soot washes into the engine oil.

NOTE: Many, if not most, engines fitted with VVT/VCT systems do not need EGR systems, since the valve overlap can be adjusted so that a small amount of exhaust gas is retained in the cylinders at the exhaust stroke. The gas that remains behind has the same quenching effect as gas that is introduced by other means.

The image below shows a typical EGR valve that is clogged up with carbon. Note that this problem is far more prevalent on diesels than on gasoline engines. When this does occur on a gasoline engine, the cause is almost certain to be related to excessive oil consumption.

What are the common causes of code P0400?

Due to the large number of EGR system designs, the causes of failures in the system are many and varied. Typical causes are shorted, corroded, burnt, or otherwise damaged wiring and connectors, but some other common causes could include the following-

• Poor crankcase ventilation due to clogged or broken PCV valves.
• Infrequent oil changes.
• Use of incorrect or unsuitable engine oil.
• Vacuum leaks in vacuum control systems.
• Exhaust leaks.
• Clogged-up catalytic converters and mufflers.
• Frequent short trips during which the engine never reaches full operating temperature. Combustion is relatively incomplete when the engine is cold, which contributes to carbon build-up.
• Excessive oil consumption due to engine wear or other defects such as leaking turbocharger seals can contribute to carbon build-up.
• Defective DPFE sensor on Ford products
• Clogged EGR gas passages.
• Defective MAP sensor.
• Defective EGR valve control solenoid.
• Ruptured EGR valve diaphragm.
• Defective EGR vacuum control solenoid.
• Failed, or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced.

What are the symptoms of code P0400?

On some applications, especially diesels, there may be no symptoms present other than a stored trouble code and an illuminated warning light. However, in some cases the symptoms of code P0400 can be severe, and the vehicle may become undriveable. Some common symptoms could include the following-

• Hard, or no starting conditions if the EGR valve is stuck in the open position, allowing unmetered air to enter the engine. This applies to gasoline engines.
• Rough idling due to exhaust gas leaking into the inlet tract, causing unstable combustion. In these cases, there may be codes relating to random misfires present as well.
• Reduced acceleration due to unstable combustion if the EGR valve opens at high engine loads.
• In some cases where exhaust gas enters the engine when it should not, the PCM may overcompensate by advancing the ignition timing. In extreme cases, this can cause detonation, which is an uncontrolled combustion event.
• Reduced fuel economy.

How do you troubleshoot code P0400?

It is important to note that diagnostic and repair procedures for any EGR related code depends on the type of EGR valve fitted, since different types of EGR valves/systems fail for different reasons. For this reason, the troubleshooting tips in this guide are divided into sections to deal with each type of EGR valve/system separately. Nonetheless, regardless of the design of the EGR system, it is recommended that the manual for the application being worked be consulted before starting a diagnostic procedure.

Vacuum controlled EGR valves/systems

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

Step 2

Inspect all associated vacuum lines for signs of damage or visible leaks. Bear in mind that not all leaks are visible, so if the vacuum lines are hard to the touch, it is more than likely that air is leaking past the joint or connection. Replace all vacuum lines as required.

Step 3

On some applications, the vacuum to the EGR valve is controlled by an electric control solenoid. Where this is the case, inspect all associated wiring for damage; look for shorted, burnt, broken, or corroded wiring and connectors. Repair all defects as required.

Step 4

If all wiring and vacuum lines check out OK, remove the vacuum line from the EGR valve and plug the open end. Attach a hand-held vacuum pump to the EGR valve and start the engine. Provided that there are no other codes and faults present that could influence the idle quality, the engine should idle smoothly.

If it does, slowly apply a vacuum to the valve to cause it to open. If the valve works, the addition of exhaust gas will cause the idle quality to deteriorate. However, this deterioration should be constant for as long as the vacuum is applied. If for instance the engine starts to run rough but then settles down to a smooth idle soon after, the EGR diaphragm is leaking, causing the valve to close.

Replace the EGR valve if this happens, or if an applied vacuum has no effect on the idle quality. If a vacuum has no effect, it is also likely that the EGR valve is clogged, and while some valves can be cleaned, replacement is always the better option.

NOTE: This vacuum test may not produce a discernable effect on some diesels, due to the large excess of air. In these cases, check to see if the EGR valve spindle moves under an applied vacuum, but note that due to the location of some EGR valves, removal of the valve may be required to verify that the spindle moves under an applied vacuum.

Step 5

If an applied vacuum produces an effect on idling, suspect a defective vacuum control solenoid. If the solenoid is electrically operated, perform continuity, resistance, ground, and reference voltage checks on all associated wiring, as well as the solenoid itself. Replace the solenoid if all obtained readings fall within specifications, or make repairs to associated wiring to ensure that all values fall within the manufacturer’s specifications.

Step 6

Clear all codes after repairs had been made, and retest the EGR system to see if the code returns. Bear in mind that several drive cycles may be required without the code reappearing before the repair can be considered as having been successful.

Electronically controlled EGR valves/systems

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

Step 2

In these applications, the EGR valve is operated by either a single solenoid or a series of linearly controlled solenoids that open the valve to the desired position is steps. In both cases, the cause of the code is more likely to be related to the control circuit/solenoids than the valve itself.

Start the procedure by performing a thorough visual inspection of all associated wiring and connectors. Look for damaged, burnt, shorted, broken, or corroded wiring and connectors. Repair all defects as required.

Step 3

If the wiring inspection does not reveal visible faults, consult the manual on the location, color-coding, function, and routing of each wire in the circuit. Perform resistance, continuity, ground, and reference voltage checks on all wiring, as well as on all relevant control solenoids. Repair wiring or replace solenoids as required. It obtained readings fall within the manufacturer’s specifications (including those of the control solenoids), suspect a clogged EGR valve, or a plugged passage in the inlet manifold.

NOTE: It is common for EGR valves and gas passages on diesel engines to be clogged. Remove the valve to check its condition. Also, be sure to check the passage that allows gas to enter the inlet manifold. If the blockage in the manifold cannot be removed by poking at it with a sharp object, it might be necessary to remove the manifold from the engine to have the blockage cleared out chemically.

Step 4

Clear all codes after repairs had been made, and retest the EGR system to see if the code returns. Bear in mind that several drive cycles may be required without the code reappearing before the repair can be considered as having been successful.

Pressure controlled EGR valves/systems

In these designs, the EGR valve is operated by back pressure from the exhaust system. In some cases, the exhaust pressure may be assisted by a spring (less often by vacuum), to move the valve spindle.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on. In this case, “intermittent faults” refer to the possibility that the valve spindle may be sticking only some of the time, thus producing erratic, sporadic, or unpredictable effects and symptoms.

Step 2

Since exhaust back pressure is the primary “power source” in these designs, even small exhaust leaks can have large effects on how well (or not) the EGR valve works. It should therefore be obvious that the exhaust system should not have any leaks. Inspect the exhaust system and have any leaks found repaired professionally to ensure that the EGR valve has the full benefit of the exhaust back pressure.

NOTE: An inspection of the exhaust system must include an inspection of the mufflers and catalytic converter(s) as well. Even partially clogged mufflers and/or converters can raise the back pressure in the exhaust system to the point where the working of the EGR valve is affected. Replace any muffler(s) or catalytic converters that are in a less than perfect condition to ensure that the back pressure always falls within specifications.

Step 3

One admittedly crude test of this type of EGR valve is to have an assistant partially restrict the exhaust tail pipe with a rag while the engine is idling. If the exhaust system does not have leaks, the increased pressure will open the EGR valve, which will affect the idling negatively. Upon removal of the restriction the idling should return to normal; if it does not, suspect a sticking valve spindle. However, if the EGR valve is vacuum assisted, inspect the vacuum lines for leaks. Repair as required, and repeat the test.

If restricting the exhaust does not produce any effect on the idle quality, suspect a clogged EGR valve or gas passages. It may be necessary to remove the EGR valve from the engine to check for blockages. If the valve itself is clogged up with carbon, replace it with an OEM part. Note that some blockages of gas passages in the inlet manifold may require chemical removal.

Step 4

Clear all codes after repairs had been made, and retest the EGR system to see if the code returns. Bear in mind that several drive cycles may be required without the code reappearing before the repair can be considered as having been successful.

Ford EGR valves/systems

Many, if not most Ford models use a DPFE (Delta Pressure Feedback) sensor to measure absolute manifold pressure and exhaust back pressure. When the PCM detects that DPFE and manifold pressure readings do not agree, or conform to specified values for a given engine speed and load, a code will be stored, and a warning light illuminated.

In practice, the DPFE sensor measures the flow rate of recirculated exhaust gas when the EGR valve is open. This rate of flow is converted into a signal voltage that the PCM uses to calculate/regulate the flow of gas in the recirculation system in order to maintain the efficiency of both the engine and the EGR system under all operating conditions. Typically, the DPFE sensor is located away from the actual EGR valve, and it is fed with exhaust gas via rubber or steel hoses that can perish, or become clogged.

When this happens, or if the sensor fails (a very common occurrence), any of the following codes can be set- P0171 & P0174, which relate to lean running conditions, and/or P0401, which indicates an insufficient EGR flow rate. Replacing the DPFE sensor will resolve these codes nine times out of every ten.

Troubleshooting Ford EGR valves /systems

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

Step 2

Perform a thorough visual inspection of all associated wiring and connectors. Look for damaged, burnt, shorted, broken, or corroded wiring and connectors. Repair all defects as required.

Step 3

If no visible defects on associated wiring are found, perform continuity, ground, resistance, and reference voltage checks on the DPFE sensor control circuit. Repair defects on the wiring as required to ensure that all readings fall within specifications.

If the wiring checks out OK, consult the manual on the correct testing procedures for the DPFE sensor, and replace the sensor if it does not comply with specified values.  In the unlikely event that replacing the DPFE sensor does not resolve the problem, follow the trouble shooting steps as outlined for vacuum assisted EGR valves.

Step 4

Clear all codes after repairs or component replacements have been completed, and retest the EGR system to see if any codes return. Bear in mind that several drive cycles may be required without the code reappearing before the repair can be considered as having been successful.

General considerations to keep in mind

• Always make sure that the PCM and other controllers are disconnected from the EGR control circuit before starting continuity and resistance checks to prevent damage to the controllers.

• The PCM gathers information from several other driveability sensors such as the MAP Sensor, Barometric Pressure Sensor, Throttle Position Sensor, and Oxygen Sensors to calculate the appropriate amount of exhaust gas to re-circulate. Thus, before attempting to diagnose P0400, diagnose and resolve these codes first to prevent a possible misdiagnosis.

• Always replace EGR valves with OEM parts to ensure that the replacement is rated for that particular application. Since replacement EGR valves may be identical in appearance to the failed unit from the outside, there may be significant differences in how the two valves respond to commands from the ECU or a vacuum feed. Two units that appear to be identical may be calibrated differently in terms of the amount of gas they allow to pass through them, or how much vacuum or pressure it takes to activate them.

Codes Related to P0400

• P0401 – Relates to “Exhaust Gas Recirculation Flow Insufficient Detected”
• P0402 – Relates to  “Exhaust Gas Recirculation Flow Excessive Detected”

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