OBD II code P0401 Exhaust EGR Flow Insufficient

It's easy to test an elecrtonically-controlled ERG valve 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.

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).

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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. Read more about the freeze frame.

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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