DTC P0401 EGR Low Flow Detected, P0402 EGR Excessive Flow Detected; Understanding, Testing, and Repairing

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P0401 and P0402 Diagnostic Trouble Codes (DTC) relate to the Exhaust Gas Recirculation (EGR) system in vehicles. Specifically they identify problems that have been detected in the flow rates of the exhaust gasses back into the engine. These exhaust gases are recirculated into the engine to lower combustion temperatures to reduce and or eliminate spark knock.

These two DTCs are opposite sides of the ‘same coin’ to use an old saying. P0401 indicates low or no flow of exhaust gases, while P0402 indicates the Powertrain Control Module (PCM) has detected excessive flow of exhaust gases.

These codes are the results of commands issued by the PCM for one of two purposes. Either through monitoring of the system through normal operations or as a test of the systems functions as part of a self-diagnostic test.

Let us discuss the EGR valve and how it functions from a generic point of view. There are two basic designs of the valves. They are powered by either vacuum or electricity. The vacuum type valve uses engine vacuum to lift the valve off its seat via a rubber diaphragm attached to the valve, all encased in a metal housing.

The electric EGR valves such as the GM type use electric solenoids attached to a plate, which is attached to the valve. These solenoids ground circuits are applied by the PCM at different times and amounts to regulate how much exhaust gas passes back into the engine.

There is a hybrid of the two designs, where a solenoid regulates the vacuum to the EGR valve. This is common on several manufacturers such as this Ford EGR Solenoid, called an EVRV (Exhaust Vacuum Regulator Valve) by Ford. This design like many others is actually a regulated vacuum leak. If we remove the cap, underneath is a foam filter covering the “regulated vacuum leak” tube.

Ford EGR Components

It does not matter which design is on the vehicle, its purpose is still the same, to introduce exhaust gasses into the combustion chamber when needed.

When the vehicle is new and these codes are displayed the causes are rather limited such as the valve or one of its components has failed, the power source has failed (broken vacuum hose or wire), sensor or the PCM needs reprogrammed.

After the vehicle has some miles on it, this all changes. Thus, our diagnostics must change. Unfortunately, these miles necessary can be as little as 20,000 miles on some vehicles. It becomes important to use a baseline diagnostic procedure such as the OBD2 Baseline Procedures Guide to identify and eliminate potential problems and not be myopic thinking of the valve or wiring is bad every time

Since this system uses exhaust gasses, it has to contain carbon and that is deposited in the passages of the engine leading to the cylinders. Some passages are located in the intake manifold; others are in the cylinder heads. These passages and the one in the EGR valve are extremely small and can become stopped up with this carbon rather easily. In addition, if the wrong fuel grade is used or too much time elapses between oil changes, it can make the problem worse very quickly.

We should always perform a quick check of the EGR valve to verify its capability to function, and does influence engine performance. This makes the diagnostics quicker and more accurate. This should include a physical measurement of the engine vacuum at idle and at 2000 rpm with a gauge; this is not the time to accept the MAP sensor readings on a Scan Tool as accurate.

An area often overlooked that can cause these codes is the exhaust itself being stopped up and constantly lifting the EGR valve off its seat as well. Depending on the amount of stoppage, it could lift the valve only during hard acceleration. This is almost impossible to detect and requires the use of a data recorder on road tests.

During your diagnostic, be sure to check the exhaust backpressure, which is a very simple test. Otherwise you may replace components only to have the vehicle back tomorrow as a recheck. This exhaust stoppage can become severe and keep the vehicle from accelerating or it could simply be enough of an annoyance to keep the light coming on with the same code.

For the Professional:

In this article, I have discussed these DTC codes in a general. Keep in mind that IF the EGR valve were already open the PCM would see no difference when it commanded the EGR to open. In addition, IF it were stuck from carbon, a small solenoid may not be powerful enough to change the valve position, thus giving the indication that a wiring problem exists when it does not.

You should never use the diagnostic procedures or specifications from one vehicle to another; this includes vehicles from the same manufacturer, model and submodel, especially with different engines. Doing otherwise most certainly will cost you money! 

Check the manufacturers’ diagnostic procedures to that vehicle using their specific auto repair manuals (data system). If you do not have their manuals, you can substitute something like Mitchell on Demand or AllData. Remember these last two can and do make mistakes in their data through combining the data for republishing.

Eg: A customer with a relatively new Expedition obtained a “Free Diagnostic” from a shop offering that service. He purchased a new EGR valve, EVRV (solenoid) and EVP Sensor. He took his vehicle in for the installation of his parts. He thought he would save money!

The parts were replaced, the codes cleared, and the customer took the vehicle. The next day he was back saying nasty things. It is understandable that he could be angry after spending several hundred dollars and not fixing the problem.

After the service adviser explained the shop simply replaced parts he furnished, and that the shop was not responsible for his misdiagnosis. He agreed to have a proper diagnosis performed. It was discovered that the left exhaust was almost completely stopped and would need to be replaced to fix the problem.

The lesson here is to perform a proper diagnosis on these codes and to test BOTH sides of the exhaust for restrictions. Also never, use the procedures from one vehicle to test the DTC of another. Even though the codes state the same thing, each vehicle has its own procedures for proper testing.


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Fortunately, everything you need everything you need to fix it right the first time is right here!

To access the sensor and data from your Engine Controller (PCM) you will need:

Professional Quality Scan Tool such as our 2X80S Scan Tool series.

Professional Quality Automotive Diagnostic Software such as OBD2 Spy, OBD 2007, or Scan Master.

If you understand how the systems were designed to work and how to test them, great! Otherwise you will need factory product service training manuals that will teach you and guide you through the diagnostic phase.

You might also need the manufacturer factory service manuals and data systems that provide specifications and details relative to your specific model.

Or aftermarket data systems such as All Data, Mitchell On Demand, Auto Data, Bosch, and others.

Everything is available here for you as a single source for all your needs. The aftermarket data systems are available in our members only area by direct shipment from the distributor at great pricing.

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