Engine Code p0401 Meaning, Causes, Fixes (4 Step Repair Guide)

Did the check engine light come on in your car? 

You're in the right place!

In this ProperMechanic.com guide, you'll learn:

  • What this engine code means
  • How to assess the code
  • How to repair the issue

And much more!

Generally, when you get a check engine light, it means you’re going to have to hook up the OBD-II scanner to see which trouble code pops up. 

Engine Code p0401 Meaning, Causes, Fixes (4 Step Repair Guide)

Read more for engine code p0401 meaning, causes, fixes in this DIY article about insufficient EGR (exhaust gas recirculation) flow. 

What Does The p0401 Code Mean? 

Okay, there's a code that says p0401; this code means that there is an insufficient flow of unburnt fuel going back into the air intake stemming from the EGR system.

Now, what is the EGR? 

Great question. It's a simple mechanism that:

  • Diverts rich exhaust gases back into the engine 
  • Helps the engine perform more efficiently
  • Cools combustion temperatures

Is it a big deal? No, it's not life and death. 

And your engine will still run, but less efficiently and rougher.

This repair doesn’t cost a lot of money in most cases, so it’s not the end of the world when this light comes on in the dashboard.

You may also notice that your vehicle has started to have a rough idle because of this error code.

The EGR valve is a great part of the engine because it helps reduce oxides of nitrogen in the atmosphere and also gives the engine a better fuel economy in the process.

When the p0401 code is pulled, it means that your engine is not running at its most full potential.

Read More >> What Does Engine Code P0304 Mean?

What Caused The p0401 code (Possible Symptoms)?

Here are a few causes of the p0401 engine code:

  • failed EGR solenoid
  • failed vacuum hose going to the EGR
  • EGR valve stuck open/shut
  • excess carbon buildup in EGR passages/EGR tube
  • failed EGR temperature sensor
  • bad intake manifold seal
  • bad EGR gasket
  • damaged wiring/failed circuitry
  • a bad engine computer

And here are a few of the symptoms that coincide with the p0401 code:

  • rough idle
  • decreased fuel economy
  • check engine light
  • engine knocking
  • failed emissions test
  • less engine power
  • engine stalling

EGR valves have a service interval of about 100 thousand miles.

Read More >> What Does Engine Code P0302 Mean?

Did you know: Other common causes of an EGR system malfunction include a faulty DPFE sensor, a catalytic converter that is clogged, EGR valve control circuit electrical issues, or a malfunctioning engine control unit.

How To Diagnose A p0401 Code?

The meaning of the p0401 code is a malfunction with the EGR valve or one of its components. So how do you know what exactly you need to do to get the problem fixed?

You’ve got to first late the EGR valve. You can do this by looking inside your vehicle's service manual.

The first natural move is to check out the EGR valve and mechanism to see what's going on. Most EGR valves are just a couple of bolts, and can easily be accessed for removal and inspection. 

What is the EGR valve?

The exhaust gas recirculation valve is a mechanism that captures unburned fuel and recirculates it back through the air intake.

With the help of a DPFE sensor and EGR temperature sensor to monitor the levels and an actuator solenoid that opens when the DPFE sensor senses unburnt fuel in the system.

Here are the essential steps for diagnosing this error.

  1. Check the Fuse/Electrical Test
  2. Remove the EGR Blots, Remove the EGR Valve
  3. Check for Signs of Carbon Buildup
  4. Vacuum Leak?

Step 1: Check the Fuse/Electrical Test

Before you go ripping the EGR off, be sure that it is getting the electricity that it needs first.

You can test all cables of the EGR by back probing them with a multimeter to see if they have the right voltage.

Shown below is an excellent video showing how to test an electrically actuated EGR valve.

Step 2: Remove the EGR bolts, remove the EGR valve

So you've tested the electrical components of the EGR. The EGR is getting all of the electricity it needs, but it's still not actuating. 

It's now time to remove the EGR and check it for signs of carbon buildup. 

This step is especially important for a diesel-powered vehicle since diesel fuel generally leaves more carbon deposits in the EGR system. 

Step 3: Check For Signs of Carbon Buildup

After removing the EGR and components, check the insides of the hoses signs of excessive carbon buildup. 

It will be pretty obvious to see because the whole inside of the unit will be caked with black gunk. 

Even if it's not the main problem, you should clean the system while you have it disassembled to avoid problems in the future. 

Step 4: Vacuum Leak?

If your EGR has an engine vacuum leak, it could throw a different code, like the OBD-II code p0400 for example.

If your system is vacuum actuated, you can use a vacuum pump to activate the valve while monitoring the engine's RPM and DPFE sensor voltage. 

The video below is a good example of an EGR valve that has a bad diaphragm seal. 

Read More >> What Does Engine Code P0420 Mean?

How Do I Fix A p0401 Code?

One thing is for certain, if your emissions test is coming up soon you are not going to pass, so this is something that you are going to want to take a look at. 

It's also just nice to not have a check engine light on the dash, as dashboards seem to turn into Christmas trees at some point. Here's how you can fix the p0401 code.

Here are the key steps to fixing a p0401 code when it pops up:

  1. Clean the EGR Valve (if necessary)
  2. Make the Electrical/Vacuum Repair (if necessary)
  3. Replace/Reinstall EGR Valve/Hoses and Gaskets
  4. Reset The Check Engine Light

Step 1: Clean EGR Valve(if necessary)

Clean the EGR system and all of its components if you feel it's necessary.

If you diagnosed the EGR valve as clogged, you must clean it out to remove the blockage that turned the check engine light on, and restore sufficient EGR flow. 

Cleaning any form of carbon deposits can be done by spraying liquids like brake cleaner/carb cleaner inside, and cleaning it with a wire brush. 

Step 2: Make The Electrical/Vacuum Repair(if necessary)

You have found the vacuum hose, vacuum pump, differential pressure sensor, electrical circuit, pressure feedback sensor, fuse, or engine computer problem already. Now it's time to make the appropriate fix/change out the part. 

This step is subject to the problem at hand. Unplug the negative battery terminal when making electrical repairs to avoid further problems. 

Each vehicle's EGR system is different. You will need to look at the vehicle's wiring diagram when making electrical repairs to verify what goes where. 

Step 3: Replace/Reinstall EGR Valve/Hoses and Gaskets

Having a copy of your vehicle's service manual will help you perform the repair by the book if you are having any doubts.

Replace/reinstall the EGR unit along with the electrical connector(s) and bolts that go to it. Remember to replace all gaskets with new ones to avoid getting the same check engine light.

Torque all bolts to spec using your vehicle's service manual as a reference.

Step 4: Reset The Check Engine Light

After you have the EGR system successfully back into place with the correct parts installed or repair performed, it's time to reset the check engine light.

Follow the steps on your OBD-II scanner for resetting the check engine light.

If you aren't using an OBD-II scanner, you can simply unplug the negative terminal of the battery for a minute or so and the light should reset on its own.

After you confirm that the check engine light is off, drive the vehicle around to ensure that it doesn't come back on. 

About Your Mechanic

About Ryan Nichols

Ryan here! My pro mechanic career began as a technician for Mercedes-Benz. After two years of that great experience, I went out on my own in both the automotive and construction fields. I've since pulled my share of salvage yard parts and fixed more vehicles than I can count.

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