P0401 – Exhaust gas recirculation (EGR) system -insufficient flow detected

By Reinier (Contact Me)
Last Updated 2017-09-22
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P0401 Mean?
2. Where is the P0401 sensor located?
3. What are the common causes of code P0401 ?
4. How expensive is it to fix code P0401 ?
5. What are the symptoms of code P0401 ?
6. What are common solutions to code P0401 ?
7. How serious is code P0401 ?
8. How safe is it to still drive the car with code P0401 ?
9. How difficult is it to repair code P0401 ?
10. What are the common mistakes when repairing code P0401 ?
11. How do you troubleshoot code P0401 ?
12. Codes Related to P0401
13. Get Help with P0401

What Does Code P0401 Mean?

OBD II fault code P0401 is a generic code that is defined as “EGR Insufficient Flow Detected”, and is set when the PCM (Powertrain Control Module) detects a lower than expected flow of exhaust gas through the EGR (Exhaust Gas Recirculation) system. Note that this code does not apply to applications that use VVT (Variable Valve Timing) or VCT (Variable Camshaft Timing) systems, since these systems achieve the same combustion temperature reducing effect as exhaust gas recirculation.

The primary purpose of an EGR system is to introduce controlled quantities of exhaust gas into the cylinders to reduce, or eliminate the formation of harmful oxides of nitrogen, which occurs at combustion temperatures of around 1500 degrees C (2800 degrees F). In a fully functional system, the controlled introduction of exhaust gas reduces combustion temperatures to below this point. However, the introduction of exhaust gas only happens when very specific operating conditions such as predefined engine temperature and engine speed have been met, but note that these conditions vary between applications.

NOTE: Typically, exhaust gas is not introduced during idling or under hard acceleration, since the introduced exhaust gas dilutes the air/fuel mixture. Thus, when exhaust gas is introduced into engine at the wrong time (or in the wrong quantities) combustion of the air/fuel mixture can be affected to the point where the quality of the engine idle and/or power delivery can suffer greatly.

When all required conditions have been met and the PCM opens the EGR valve, the manifold pressure should change by an amount that corresponds directly with the degree to which the EGR valve is opened. When this happens, the PCM makes suitable adjustments to fuel delivery, ignition timing, and other settings to ensure that the engine operates at optimum levels when exhaust gas is being introduced to counter the dilution of the air/fuel mixture.

Therefore, when the change in manifold pressure does not agree with the amount the PCM expects to see, the PCM interprets the condition as “Insufficient Flow”, and it will set code P0140 as a result. Depending on the application, a warning light may be illuminated on the first failure, or only after a preset number of failure cycles had occurred.

Where is the P0401 sensor located?

EGR valves are almost always located on, or close to the inlet manifold, and can most commonly be recognized by a steel pipe that runs between the valve and the exhaust system/manifold. In the case of Ford applications, the PDFE sensor (the failure of which is a common cause of this code) is located on the left hand (driver’s) side of the firewall.

The image below shows the typical appearance of an EGR valve that is so clogged with carbon that the flow of exhaust gas through the valve is inhibited, or sometimes prevented.

What are the common causes of code P0401 ?

Some common causes of code P0401 could include the following-

• Defective EGR valve
• Defective EGR valve position sensor
• Defective DPFE sensor/hoses (Ford)
• Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
• Damaged, split, cracked, dislodged, or perforated vacuum hoses/actuator actuator diaphragm that cause insufficient vacuum to operate the EGR valve actuator
• Defective EGR vacuum switching valve (Toyota)
• Clogged EGR passages
• Clogged EGR valve
• Failed or failing PCM, Note that this is a rare event, and the fault must be sought elsewhere before any control module is replaced

How expensive is it to fix code P0401 ?

While repair/replacement costs of EGR valves vary widely between both applications and geographical location, the average cost of replacement can be as low as $250 on the low end of the scale, to about $330 or so, on the high end. Note that these estimates include parts and labor, but do not include fees and taxes that can increase these dollar amounts significantly.

What are the symptoms of code P0401 ?

Some common symptoms of code P0401 could include the following-

• Stored trouble code, and possibly an illuminated warning light
• Depending on the type of EGR valve fitted to the application, there may be additional codes present along with P0401
• Some applications may exhibit a profound “knock” upon acceleration
• On some applications, there may be no symptoms at all except for a stored code and a warning light.
• Fuel consumption may increase on some applications
• Vehicle may fail an emissions test due to high nitrous oxide levels in the exhaust

What are common solutions to code P0401 ?

The most common solutions to code P0401 could include the following-

• Replacement of clogged EGR valves
• Inspection and repair of defective wiring
• Inspection and repair of vacuum control systems
• Clearing out of clogged EGR passages in the manifold, and sometimes in cylinder heads

How serious is code P0401 ?

Code P0401 should be considered as serious, since the combination of high combustion temperatures and pre-ignition (knocking or pinging- if present) over extended periods can cause severe damage to pistons and valves. This code should therefore be resolved in a timely manner.

How safe is it to still drive the car with code P0401 ?

This code does not pose a safety risk, unless the application has severe driveability issues. Note however, that it is relatively rare for P0401 to cause serious drivability issues.

How difficult is it to repair code P0401 ?

Non-professional mechanics should take note that while diagnosing this code is easy and relatively straightforward, fixing the cause(s) can vary from being easy, to being extremely challenging. Note that in serious cases, it may be necessary to remove the intake manifold to clear out EGR passages, while in a few extreme cases, it may be even be necessary to remove the cylinder head(s) in order to clear out EGR passages.

However, in most cases the most common remedy involves removing the EGR valve for either cleaning, or replacement. Provided some basic tools are available, this procedure should not present the average non-professional mechanic with undue difficulties.

What are the common mistakes when repairing code P0401 ?

The most common mistake(s) made when diagnosing/repairing this code is replacing the EGR valve without that all EGR passages are open, or verifying that the EGR valve’s control circuit is functioning properly.

Other common mistakes include replacing the EGR valve with substandard, aftermarket replacements, or replacing EGR valve’s with incorrect or unsuitable replacements. Bear in mind that while some EGR valves for any given model range may appear to be identical on the outside, EGR valves are calibrated and ported differently to accommodate the EGR requirements of the particular engine it is designed to work on.

How do you troubleshoot code P0401 ?

SPECIAL NOTEs: The most common cause of this and several other EGR related codes on Ford applications is a defective DPFE (Delta Pressure Feedback) sensor. This sensor uses a pressure differential across an orifice in the exhaust gas feed pipe to regulate EGRG flow, and while failure of this sensor/system is a known cause of code P0401 on Ford applications, it is often overlooked or ignored. Therefore, a diagnostic procedure for this code on Ford applications should ALWAYS begin by inspecting and/or testing of the DPFE sensor and its associated hoses in order to avoid a misdiagnosis. END OF SPECIAL NOTES.

NOTE #1: Due to the large number of different types of EGR valves in use today, it is not possible to provide detailed diagnostic and/or repair information here that will be accurate for all applications under all conditions. Nonetheless, the few generic diagnostic and repair steps outlined here will resolve this code in nine out of every ten instances of this code on most applications, provided proper reference is made to the manual for the affected application at each step.

NOTE #2: If the EGR valve on the affected application is vacuum operated, a hand-held vacuum pump that is fitted with a graduated gauge will be most helpful to diagnose this code.

Step 1

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

NOTE: If other codes are present along with P0401, note the order in which they were stored, and repair or resolve all additional codes strictly in this order. Failure to do this will almost certainly result in a wrong diagnosis, and the unnecessary replacement of parts and components. Moreover, resolving codes that precede P0401 may very resolve P0401 at the same time.

Step 2

If there are no additional codes present, refer to the manual for details on the type of EGR valve that is fitted to the application, but DO NOT begin to test ANYTHING until you have clear understanding of the type of valve, and the operating principles of the control system. Failure to understand how the EGR system works on the affected application will make it almost impossible to diagnose the problem accurately.

Step 3

Next, locate the EGR valve and its associated wiring or vacuum control lines. If the EGR valve is electrically operated, determine the function and color-coding of each wire in the connector to avoid testing the wrong circuit later on.

If the EGR valve is electrically operated, use the scanner to command the valve open and closed multiple times. The scanner will display the status of the valve: if the valve’s status does not change, perform a thorough visual inspection of all associated wiring. Look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors.

If the wiring is free of visible damage, perform resistance, ground integrity, continuity, and reference voltage (if applicable) on all wiring, as well as the EGR actuator itself as per the instructions in the manual. Compare all obtained readings with the values stated in the manual, and replace the EGR valve or make repairs to wiring as required before testing the valve again to verify that it works as designed.

NOTE #1: Many EGR valves are fitted with position sensors that relay the position of the valve pintle to the PCM. Failure of this position sensor or its signal circuit are common causes of P0410, so be sure to test the sensor and its signal circuit along with the other wiring.

NOTE#2: Some EGR valves are fitted with temperature sensors that relay the temperature of the exhaust gas/intake air to the PCM. Failure of this sensor or its signal circuit are common causes of P0410, so be sure to test the sensor and its signal circuit along with the other wiring.

Step 4

If the EGR valve now works, start the engine and allow it to idle. Command the valve open with the scanner. If all the EGR passages are open, and everything else in the EGR system works as designed, the idle quality will deteriorate immediately. This is normal, and indicates that the EGR system is functional. If however, the EGR valves’ status changes on the scanner, but the engine idling is not affected, suspect clogged EGR passages, or carbon build-up in the EGR valve that inhibits or prevents the flow of exhaust gas.

Step 5

Allow the engine to cool down, and remove the EGR valve to inspect it for carbon deposits. If the valve is clogged up with carbon deposits, resist the temptation to clear it out, since this could damage the valve beyond repair. In these situations, the better option is always to replace the valve with an OEM replacement.

If however, the valve is clear of carbon, refer to the manual for details on how to proceed to clear out the EGR passages. If you have any doubts about your ability to do this successfully, the better option is to refer the vehicle to the dealer or other competent repair shop for professional assistance to prevent damage to the manifold or other expensive components.

Step 6

If the EGR valve is vacuum operated, perform a thorough visual inspection of all associated vacuum lines. Look for dislodged, split, cracked, or perforated lines, and replace all vacuum lines that are in a less than perfect condition. Once repairs are complete, operate the vehicle normally before rescanning the system to see if the code returns.

Step 7

If the code persists, attach the vacuum pump to the EGR valve actuator. Allow the engine to idle while drawing a vacuum in the actuator. If the actuator works, the engine idle will deteriorate immediately; this is normal, and indicates that the EGR system is functional. However, the actuator must hold the vacuum for at least several minutes without decaying. If it does decay, however slowly, the actuator is defective and the entire EGR valve assembly must be replaced with an OEM replacement to ensure proper operation.

Step 8

If the vacuum holds but the engine idle does not deteriorate, suspect clogged EGR passages, or carbon build-up in the EGR valve that inhibits or prevents the flow of exhaust gas. Allow the engine to cool down, and remove the EGR valve to inspect it for carbon deposits. If the valve is clogged up with carbon deposits, resist the temptation to clear it out, since this could damage the valve beyond repair. In these situations, the better option is always to replace the valve with an OEM replacement.

If however, the valve is clear of carbon, refer to the manual for details on how to proceed to clear out the EGR passages. If you have any doubts about your ability to do this successfully, the better option is to refer the vehicle to the dealer or other competent repair shop for professional assistance to prevent damage to the manifold or other expensive components.

Codes Related to P0401

Note that while the codes listed below are not strictly related to P0401 – “EGR Insufficient Flow Detected”, any of the codes below, or a combination of them can conceivably cause code P0401, or contribute to its setting.

• P0400 – “Exhaust Gas Recirculation Flow Malfunction”
• P0402 – “Exhaust Gas Recirculation Flow Excessive Detected”
• P0403 – “Exhaust Gas Recirculation Circuit Malfunction”
• P0404 – “Exhaust Gas Recirculation Circuit Range/Performance”
• P0405 – “Exhaust Gas Recirculation Sensor A Circuit Low”
• P0406 – “Exhaust Gas Recirculation Sensor A Circuit High”
• P0407 – “Exhaust Gas Recirculation Sensor B Circuit Low”

• P0408 – “Exhaust Gas Recirculation Sensor B Circuit High”

• P0409 – “Exhaust Gas Recirculation Sensor “A” Circuit”
• P040A – “Exhaust Gas Recirculation Temperature Sensor “A” Circuit”
• P040B – “Exhaust Gas Recirculation Temperature Sensor “A” Circuit Range/Performance”
• P040C – “Exhaust Gas Recirculation Temperature Sensor “A” Circuit Low”
• P040D – “Exhaust Gas Recirculation Temperature Sensor “A” Circuit High”
• P040E – “Exhaust Gas Recirculation Temperature Sensor “A” Circuit Intermittent/Erratic”
• P040F – “Exhaust Gas Recirculation Temperature Sensor “A”/”B” Correlation”

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