P0401 – Exhaust Gas Recirculation “A” Flow Insufficient Detected

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When the P0401 OBD2 code activates, it tells you there’s an issue in your exhaust gas recirculation system. Problems with this system increase the harmful emissions in your exhaust and cause you to fail an emissions test.

Resolving this problem is often a simple matter, though you may need to go through several steps to diagnose the trouble code source properly. We’ll go through what this code means and what steps to take to clear it below. 

P0401 Code Definition

Insufficient Exhaust Gas Recirculation (EGR) Flow

What Does P0401 Mean?

The exhaust gas recirculation (EGR) system redirects some carbon dioxide from the exhaust to the engine’s combustion chambers. This process lowers the combustion temperature and limits how many smog-forming nitrogen oxide compounds get produced. 

If the EGR system sends the wrong amount of carbon dioxide back to the engine or releases it at the wrong time, the combustion temperature won’t be lowered enough. When this happens, nitrogen oxide production increases, and the P0401 trouble code activates. 

There are three parts of the EGR system that work together when it’s functioning correctly: the EGR valve, the actuator solenoid, and the differential pressure sensor. If any of these are clogged or failing, it could cause the P0340 OBD2 code to activate. 

What Are The Symptoms Of The P0401 Code?

  • Activation of the check engine light
  • Reduced acceleration power
  • Pinging or knocking sounds in the engine on ignition or acceleration
  • Failed emissions test due to increased nitrogen oxide production

What Are The Causes Of P0401?

The cause of the P0401 trouble code can be one of the components of the EGR system, such as:

  • EGR passages: The passages may be clogged with carbon build-up or other debris blockages, restricting the flow of carbon dioxide into the engine and triggers the P0401 code. 
  • EGR valve: The valve could be faulty or stuck. It may also fail to open because the system is not maintaining a vacuum.
  • EGR sensor: Carbon build-up can coat the EGR sensor itself and cause it to malfunction. The sensor could also be misreading the temperature change when the valve is opened because it’s faulty or damaged.

How Serious Is The P0401 Code?

The P0401 OBD2 code is moderately severe. Even though the drivability issues are usually mild, ignoring problems with the EGR system can damage the engine’s valves and pistons. You should identify and fix the cause of this trouble code as soon as you notice it. 

How To Diagnose The P0401 Code

Tools you’ll need:

  • OBD2 scanner
  • Throttle body cleaner
  • Carburetor cleaner
  • Vacuum pump
  1. Inspect the hoses and wires connected to the EGR system. Replace any that are corroded or damaged and check for blockages that could interrupt the airflow. 
  2. Check the EGR sensor and EGR valve for carbon build-up. Use throttle body cleaner to remove the build-up.
  3. Test the diaphragm inside the EGR valve for leaks. Put on your parking brake and block your wheels, then start your engine. Insert the straw of a can of carburetor cleaner into the valve and spray. The engine’s RPM should stay consistent. If it increases, the diaphragm is leaking, and you should replace the valve. 
  4. Use a vacuum pump to verify that the EGR system is maintaining a vacuum. Please turn on your car and idle the engine until it reaches operating temperature. Disconnect the vacuum hose from the EGR valve and plug the hose with a screwdriver. Connect the vacuum pump to the hose opening on the valve and apply 15 Hg of vacuum. The engine should idle rougher and may even stall when you use the vacuum, and you should see the valve diaphragm moving. If the plunger doesn’t move, the valve is defective. If the plunger does move, but there’s no change in the idle, there are likely carbon deposits blocking the EGR passages. 
  5. Remove the intake manifold and check it for blockages and build-up. Also, remove the hoses that connect to the manifold and inspect them. Clean any carbon deposits you find using throttle body cleaner, then re-install all the components and ensure they’re attached securely.

Common Mistakes To Avoid While Diagnosing The P0401 Code

Many people instantly replace the EGR valve when the P0401 trouble code activates, but this valve isn’t always the problem’s source. Make sure you go through every step of the diagnosis above before deciding which components need replacing. The EGR sensor is as likely to be the problem as the valve itself. Check all the hoses, passages, and parts of the system for blockage and damage before jumping to any conclusions.

What Should You Do To Fix The Code P0401?

When making repairs, make sure to clear the codes and re-scan your system after every step to see if the P0401 code has been removed. 

  1. Replace any damaged wires or hoses discovered in your diagnosis.
  2. Remove the EGR valve and clean it using throttle body cleaner. While it’s removed, inspect the passages around the valve for blockages and remove any you find.
  3. Remove and clean the EGR sensor, then re-install it.
  4. Replace the EGR valve if it continues to fail steps 3 and 4 in the diagnosis after cleaning. 
  5. Test that the EGR sensor is receiving power using a multimeter. If it fails, replace your sensor.
  6. Clean the intake manifold and all associated hoses.
  7. If the code still hasn’t cleared, the problem is likely a deeper electrical issue. Take your car to a mechanic for further diagnosis. 

Tips To Avoid P0401 In The Future

Preventing clogs and blockages in the EGR system is the best way to avoid the P0401 OBD2 code. You can buy caps for your EGR valve to prevent carbon build-up. These aren’t recommended for all vehicles, so ask your mechanic before taking this step. 

Regular cleaning of your EGR system can prevent clogs from reaching the point that it activates the P0401 trouble code. Keep an eye on your emissions using an OBD2 scanner. When the harmful components in your exhaust increase, use throttle body cleaner to clean out your EGR system as described above.