OBD II P0401 Insufficient EGR flow

Information about the P0401 fault code, what can and cannot cause it to occur, including technical information to aid in the understanding of this code. This information is from our emissions expert and includes diagnostic procedures that can be taken to a shop if they are having problems diagnosing this code.

OBD II Fault Code:
P0401

Fault Code Definition:
Insufficient EGR flow

Symptoms:
Check Engine Light will be illuminated
In some cases there may be no adverse conditions noticed by the driver
In other cases there may be performance problems, such as pinging on acceleration, when the engine is under load or when driving the vehicle at higher speeds.

Common Problems that Trigger the P0401 code:

• Restriction in the EGR passages, usually caused by carbon buildup
• Lack of proper vacuum or electrical signal to the EGR valve
• Lack of proper EGR system feedback to the computer from the:
     Manifold Absolute Pressure Sensor (MAP)
     Differential EGR Pressure Feedback Sensor (DPFE)
     EGR Valve Position Sensor (EVP)
     EGR Temperature Sensor
• The EGR Valve is defective

Watch this video for an understanding of the EGR system:

The Basics:
NOx gases are formed when the combustion temperature is too high (2500°F) EGR Systems are used to reduce the combustion temperature, thus reducing NOx formation.

The exhaust gas recirculation (EGR) system recycles a small amount of exhaust gas from the exhaust system (usually no more than 10 percent) and mixes it with the intake manifold air entering into the combustion chambers. The addition of this inert (or non-combustible) exhaust gas limits the peak combustion temperatures to a range that is below 2500° F, where the formation of nitrogen oxide (NOx) is know to occur. In some cases where the engine is pinging and/or knocking badly from a severe lack of EGR flow, misfires can take place which allow raw Hydrocarbons (HC) to be released from the tailpipe.

P0401 Diagnostic Theory for Shops and Technicians
When the computer sets a code P0401, it means that the EGR flow monitoring criteria has not been met. The EGR monitoring criteria are a set of test values and are usually run during at least 2 different driving conditions such as steady speed freeway driving and steady speed city driving.

The engine control module determines proper EGR flow in many ways:

• temperature increase in the EGR passages when the EGR is supposed to be flowing
• measurable amount of Manifold Pressure change when the EGR is supposed to be flowing
• measurable change (usually a decrease) in the front Oxygen Sensor Signal
• position change in the EGR valve as measured by an EGR Valve Position Sensor
• amount of Spark Knock as measured by the Knock Sensor
• amount of decrease in exhaust back pressure as measured by the Digital EGR Pressure Feedback Sensor

The code P0401 is usually NOT a problem with the EGR valve itself. It means that the EGR system is NOT allowing enough EGR to flow back in to the combustion process to sufficiently cool the peak firing temperatures. Once the code P401 has been retrieved with a scan tool the Freeze Frame data should be documented and analyzed. This is to determine what engine conditions were present when the code was triggered. It is recommended that the vehicle be driven in such a way as to duplicate the code setting conditions with a data streaming scan tool connected, so the behavior of the EGR operating components and feedback sensors can be monitored.

There are several tests for EGR systems to isolate whether it is an EGR control problem, a plugged or restricted system or a defective feedback device. Here are some of the most common:

• Does the engine die when the EGR valve is manually raised to its maximum?
  (Either with a vacuum pump or with a bi-directional scan tool when it’s a digital EGR valve)
• Is the EGR valve getting sufficient vacuum? (use the manufacturer EGR vacuum spec)
• Is the EGR system restricted? (The engine stumbles but does not die)
• Is the EGR system plugged? (engine RPM does not change)
• Does the EGR valve work?
• Raise the RPM to 3000 and check manifold vacuum, then open the EGR valve to its maximum and the manifold vacuum should drop by at least 3"of mercury. If it does not, there is a flow and/or restriction problem. Also do this test and verify that the MAP sensor change is proportional to the change in vacuum
• Test the EGR temperature sensor (if equipped)with a propane torch and an DVOM
• Test the accuracy of the EGR valve position sensor with a scan tool or DVOM by raising or lowering the EGR valve
• Test the Digital EGR Pressure Feedback Sensor (DPFE) with a data streaming scan tool and verify that the voltage or lift percentage changes according to spec
• Verify that the front Oxygen Sensor readings drop and that Short Term Fuel Trim increases when the EGR valve opens. (EGR leans out the mixture)

Note: if the NOx goes DOWN when the EGR valve is raised ( this test is most commonly performed on a Dynamometer ) this usually indicates that 1 or more EGR passages to 1 or more cylinders are plugged or very restricted and all of the EGR is going to 1 or 2 cylinders. When this occurs you may notice misfires and even have misfire codes with the P0401. This can occur on some Honda vehicles as well as other vehicles that use EGR ‘runners’ for each cylinder.

» Do you have feedback on this information?