P0401 is a very common OBDII trouble code. It has to do with the emissions system and stands for:
P0401: EGR → Insufficient Flow
The EGR system on a vehicle is responsible for re-circulating exhaust gasses in order to lower the vehicles emissions output. There are three main parts to the EGR system. They are the EGR Valve, Differential Pressure Sensor, and the Actuator Solenoid.
P0401 is a general code, which means that regardless of the year (1996+ anyway) and the model, it’ll have the same meaning regardless of which manufacturer built the vehicle.
Most of the time there are not going to be symptoms when P0401 is triggered, other than the service engine soon light itself. In some extreme cases, you may notice a slight ping or knock.
- Service Engine Soon Light– P0401 will illuminate the service engine soon light.
- Knocking Sound– In certain cases, there may be a noticeable knock that comes from the vehicle. It’ll sound almost like piston slap.
Diagnosing what is causing the P0401 code is typically not very tricky. Here are the most common causes of P0401:
- Blocked EGR Tube– The tube that carries the exhaust gases back to the motor may clog up over time. Alternatively, it can become bent or kinked. This is especially true if you are experiencing P0401 right after doing some engine work. Check the tube and see if it is cracked, clogged or otherwise damaged.
- EGR Valve Itself is Bad– The EGR valve itself is usually not going to be the place to start. But, they certainly are the cause of P0401 often enough.
- Vacuum Issues– The EGR valve relies on engine vacuum to operate. So, if you are encountering P0401 and other codes, the P0401 may be a symptom of larger problems. For instance, if the motor is not running well enough to create sufficient vacuum to operate the EGR valve, than P0401 will often be thrown.
- Sensor Issue– The Differential Pressure Feedback- EGR sensor can go faulty and cause the EGR valve to stop operating properly and throw the code.
The good news with P0401 is that it rarely ever costs much money to fix, and it is usually pretty easy to get the problem, since it is right on top of the engine. Most folks see P0401 Insufficient EGR flow, look up the price of an EGR valve, see how cheap it is and just throw a new one on.
That’s often a mistake. That’s because the EGR valve is not going to be the culprit most of the time. Here are some of the most common fixes to help clear the P0401:
- Check the Tube- It’s a good idea to check the tube going to the EGR because it’s often the cause of P0401, easy to do, fast, and replacing it is very affordable. If it’s cracked, you’ll need to get a new one. If it’s clogged you can get away with cleaning it and replacing it.
- Check DPFE Sensor Voltage- If you are good with a voltage meter go ahead and test it. Here’s a good article on how to do that from It Still Runs: How to Test DPFE Sensor.
- Test the EGR Valve- A vacuum gauge will help you determine if the EGR valve itself has gone bad. Here’s an article from Autozone on how to test it with a vacuum gauge.
Testing your EGR system is relatively easy to do. The most challenging aspect is the DPFE sensor. If you aren’t sure you can tackle that, you can always test everything else first. By process of elimination, you can be relatively confident that the DPFE is at fault. If there is anything you would like to add about P0401, please feel free to add a comment below. Good luck!!