Rick's Free Auto Repair Advice

P0400 code — What it means and how to fix it

Learn what a P0400 code means and how to fix it

A P0400 code is defined as Exhaust Gas Recirculation Flow Malfunction; meaning that the EGR valve isn’t responding properly to the EGR flow command issued by the ECM. More specifically, it could mean the valve is closed when it should be open or vice-versa or that the EGR passage is restricted.

It’s a generic code that indicates that the engine control module (ECM) has detected that the exhaust gas recirculation isn’t what the ECM expected to see. The EGR system is responsible for diverting a certain amount of exhaust gas to the intake manifold air to reduce combustion chamber temperatures and reduce NOx formation. For more information on the EGR system and why it’s needed, see this more in-depth post.

The most common causes of a P0400 code

• Complete EGR valve failure — not opening, not reporting an opening
• Carbon buildup on the EGR valve seat or pintle that’s preventing full exhaust flow.
• Carbon buildup in the intake manifold or the EGR recirculation tubing that runs from the exhaust port to the EGR valve or from the EGR valve through the intake
• Defective EGR Valve Position Sensor — The EGR position sensor reports how far open the valve is. The computer compares the commanded opening to the data from the sensor and sets a code if the sensor opening value doesn’t match the commanded opening. In many cases, it’s just a failed sensor that sits on top of the EGR valve. This is especially common on Chrysler vehicles.
• Defective EGR Valve Position Sensor connector and wiring — Since the EGR valve is recirculating very hot exhaust gasses, the heat may have damaged the electrical connector. So check that first, before replacing the valve.
• Defective DPFE (Differential Pressure Feedback EGR) Sensor and/or Wiring — This is a common failure on Ford products that don’t use a position sensor and instead use a Differential Pressure Feedback Sensor. Ford DPFE sensors have a high failure rate.
• Lack of proper vacuum or electrical signal to the EGR valve — This is more common on older vehicles that use vacuum-operated EGR valves; vehicles from the 80’s and 90’s. Modern EGR valves are electronic

The most probable causes of the P0404 code are related directly to the EGR valve itself. It could be faulty or there might be a bad connection. Here are some of the most common causes of the P0404 code:

Here are some steps to diagnose and fix the P0400 code

• Check the EGR valve for proper operation — Remove the valve and check for carbon buildup and check the passages in the intake for restrictions.
• Connect an OBD-II scanner to the vehicle’s diagnostic connector and read all stored data and error codes.
• Clear the error codes from the computer memory and test-drive the vehicle to see if code P0404 appears again.
• Inspect the EGR valve position sensor and wiring for damage.
• Inspect the differential pressure feedback electronic (DPFE) sensor and wiring for damage.
• Check the wiring and connectors for damage or looseness.
• Replace the EGR valve position sensor if it is faulty.
• Replace the differential pressure feedback electronic (DPFE) sensor if it is malfunctioning.
• Repair or replace any damaged or loose wiring or connectors.

If the problem persists, the ECM may be faulty and require replacement.
© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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