OBD II code P0401 Exhaust EGR Flow Insufficient

Ford EGR valve
Ford EGR valve

The OBDII fault code P0401 means that the engine computer has detected that the Exhaust Gas Recirculation (EGR) system flow is insufficient. Read more how the EGR system works below.

What could cause the code P0401

- clogged or restricted EGR passages
- faulty EGR valve
- EGR valve stuck closed
- problems with the DPFE sensor (Ford)
- problems with hoses to the DPFE sensor (Ford)
- problem with the EGR vacuum switching valve (Toyota)
- clogged catalytic converter
- carboned up EGR temperature sensor
- vacuum supply issues at the vacuum-operated EGR valve
- electrical problems with the EGR valve control circuit
- engine computer problems


With this fault code, a vehicle might have some driveability issues, such as surging, lack of power, poor fuel economy, rough idle, or stalling. You might notice the engine knocking or 'pinging' noise (detonation) or there could be no symptoms at all.

How to diagnose

Don't start by throwing parts at it; this is one of those codes that will come back if not diagnosed properly. If you want to take it to a mechanic, have the diagnostic done at the dealer; they know common problems and have the latest repair information. If an engine computer needs to be reprogrammed as a part of the repair, your dealer can do it too.
If you want to diagnose yourself, check for common problems first - read below.

Common problems:

Clogged-up EGR valve
Clogged-up EGR valve

A clogged-up or sticking EGR valve is very common in many cars, SUVs and pickup trucks, see the photo. The EGR passages could be restricted too. The EGR ports in the throttle body clog up very often.

Ford: One of the common problems with many Ford vehicles that can cause the trouble code P0401 is a bad or damaged DPFE sensor (DPFE stands for Delta Pressure Feedback EGR). When diagnosing the code P0401 on a Ford vehicle, the DPFE sensor and the hoses to the DPFE sensor are usually tested first. The DPFE sensor and its hoses should hold the vacuum; the hoses should not be cracked or restricted. The sensor signal voltage should change when the vacuum is applied. Sometimes the DPFE sensor hoses could be reversed during a repair and this also could cause the code P0401. Often, replacing the DPFE sensor with an updated part could solve the problem, however, other components of the EGR system also need to be checked. This thread shows EGR system components in the 2.3L Ford Ranger.
If you are replacing the DPFE sensor, make sure the hoses are installed correctly and not crossed.

Honda/Acura: The code P0401 is common in Honda V6 engines. A Honda service bulletin for some 1998-2001 3.0L V6 Honda Accord models describes a problem with a clogged EGR port causing the code P0401 or P1491. Another bulletin for 1999-2002 Odyssey as well as some 2003 Honda Pilot models describes the same issue with the 3.5L V6 engine causing hesitation or surge during light acceleration with the code P0401 or P1491. The repair is not easy; it involves removing the intake manifold, cleaning the EGR port, drilling out the EGR port with a special drill bit (included in the kit), installing a new EGR pipe kit and replacing the EGR valve. Some V6 Acura models also had similar issues. See some photos at this post and here.
- Another Honda bulletin describes the problem with hesitation or stumble under light acceleration between 1,500-2,000 rpm in the 1990-1993 Accord and 1997-2001 Prelude with a 4-cylinder engine. The code P0401 is listed among other possible codes. The repair again involves cleaning the EGR ports.

Toyota Camry/Corolla: In Toyota Camry with the 2.2L engine, the code P0401 was often caused by a bad VSV (Vacuum Switching Valve) for the EGR, located under the intake at the back of the engine. In many cases, replacing this valve could solve the problem, however the EGR passages need to be checked for restrictions and carbon buildup and the EGR valve needs to be checked for sticking. This article describes the repair procedure.
- A faulty VSV (Vacuum Switching Valve) for the EGR can also cause the code P0401 in older Toyota Corolla.

Mazda Protege: In Mazda Protege, rough idle with codes P0401 and P0300 could be caused by a stuck open EGR valve. Replacing the EGR valve with an updated part often solves the problem.

What needs to be checked with the code P0401:

The most common problems with the EGR system is a faulty EGR valve and carbon deposits restricting the EGR flow. The EGR valve needs to tested. The EGR passages need to be checked for carbon buildup and restrictions.
It's easy to check the electrically-operated EGR valve for restriction with a scan tool: when the EGR valve is commanded open on a scan tool with the engine idling, the engine should stumble, run rough or even stall. If commanding the EGR valve open has no effect on the engine idle, the EGR system is plugged up or the EGR valve is inoperative or stuck closed.
If there are signs of carbon buildup anywhere in the EGR system, usually the whole system needs to be cleaned thoroughly. Many mechanics recommend replacing the EGR valve as well, as even if cleaned, the valve might stick. The electrical connectors and wiring at the EGR valve or sensor also needs to be checked for damage or corrosion. The EGR sensor, whether it's a DPFE sensor, EGR temperature sensor or EGR boost sensor need to be checked too. If a clogged catalytic converter is suspected, the exhaust back pressure needs to be checked.

Questions and answers

Q: 2001 Toyota Camry 2.2L with the code P0401. I have replaced the VSV valve but the code P0401 came back in a few days. What else do I need to replace?

A: Check the vacuum hoses at the VSV valve, check and clean the filter in the EGR vacuum modulator, check the EGR valve and passages for blockage, see if the vacuum applied directly to the EGR valve causing the engine running at idle to start running rough or stall. The electrical part needs to be checked too.

Q: 1999 Ford Explorer. The Check Engine light came on, we scanned the truck at Autozone, the code was P0401. My dad replaced the EGR valve and disconnected the battery to reset code, but the Check Engine light came back next day while driving on a freeway. The code again was P0401. What else need to be replaced?

A: Test the DPFE sensor (read above) and check EGR passages in the intake plenum and in the throttle body; they are very common to clog with carbon deposits. You might need to pull upper intake to find the blockage. If you find the blockage, all EGR passages need to be cleaned.

Q: Code P0401 in Chrysler Pacifica. This code came a month ago and my mechanic has tested the EGR system and everything was working properly. Three days ago the same code came back again. Are there any common issues that could cause this code?

A: Bad EGR valves were fairly common in many Chrysler vehicles. Of course the whole EGR system needs to be checked thoroughly to diagnose the issue, but if everything check out OK, you might want to try replacing the EGR valve.

How Exhaust Gas Recirculation (EGR) system works

The Exhaust Gas Recirculation (EGR) system is a part of the vehicle emission control system. The main purpose of the EGR system is to reduce the amount of the nitrogen oxides (NOx) in the exhaust emission. The nitrogen oxides are formed as a result of the combustion in the engine cylinders. The formation of the nitrogen oxides increases dramatically at higher combustion temperatures (above 1600 °C or 2900 °F).

The EGR system reduces the combustion temperature by diverting a small portion of the exhaust gases back into the intake manifold. The exhaust gases are no longer combustible, therefore diluting the air/fuel charge with exhaust gasses reduces combustion temperature.
An excessively-high combustion temperature is also harmful for the engine, as it may cause a detonation (pinging or spark knock), which in turn can result in blown head gasket and burned pistons. Not all vehicles are equipped with an EGR system; many newer cars utilize a variable valve timing and other means to control the NOx emissions.

How the EGR system flow is controlled: The flow of EGR gases is controlled by the EGR valve. In some cars, the EGR valve is operated by a vacuum actuator, as in this diagram (see the example). Other cars may have an electric solenoid- or electric step motor-operated EGR valve (see the example). The EGR valve is closed when the engine is cold, at idle, or during hard acceleration. The EGR flow is at its maximum during steady cruising. For this reason, when diagnosing the EGR-related OBD II codes it is important to check the freeze frame, as it contains the data about the conditions (e.g. vehicle speed, engine temperature, load) when the fault was detected.

EGR system diagram

EGR system simplified diagram

How the EGR system flow is monitored: The operation of the EGR system and the EGR flow is controlled by the engine computer or ECM. A computer system in all modern cars has a self-diagnostic and reporting capability or what is known as the on-board diagnostic or OBD II. The ECM in an OBD II compliant vehicle constantly monitors and periodically tests the EGR system along with other emission control systems.

There are different ways to monitor the EGR flow. Many cars use an EGR temperature sensor installed in the intake part of the EGR system (see the left diagram below). When the EGR valve opens, the temperature on the intake side rises from the hot exhaust gases. Ford uses a DPFE sensor (DPFE stands for Delta Pressure Feedback EGR) that measures the EGR flow based on the difference in pressure on both sides of the metered orifice in the exhaust part of the EGR system (the right diagram below). If a fault with the EGR system is detected, the ECM turns on the "Check Engine" or in OBD II terms, Malfunction Indicator Light (MIL), storing the corresponding trouble code in the memory. Most common EGR system trouble codes are the P0400 - Exhaust Gas Recirculation Flow Malfunction and P0401- Exhaust Gas Recirculation Flow Insufficient.

EGR system diagram 1 EGR system diagram 2

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