Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.
A P0401 code may come from EGR system problems like:
- Carbon buildup
- A loose or damaged vacuum hose
- A fault in an electrical circuit
- A mechanical failure
It depends on the type and configuration of your vehicle's EGR system and the nature of the fault.
With a bit of information and troubleshooting, though, you may be able to get to the source of the problem and probably solve it on your own. That's because generally, a P0401 trouble is mostly caused by a few common issues.
Also, check the Resources at the bottom of this post. You'll find links to other posts that may help you troubleshoot and fix a P0401 code.
In This Article
- What Does a P0401 Code Mean?
- What Causes a P0401 Code?
- Common P0401 Code Symptoms
- How Does an EGR Valve Work?
- How to Repair a P0401 Code
- Is a P0401 Code Bad?
- How to Clear a P0401 Code
1. What Does a P0401 Code Mean?
Basically, a P0401 code means the car's computer has detected insufficient exhaust gas flowing back into the combustion chambers. In other words, the computer knows there's a problem in the EGR system or passages but doesn't know why.
The EGR (exhaust gas recirculation) system is in charge of recycling a controlled amount of exhaust gases back into the cylinders to reduce combustion temperatures and thus, reduce highly toxic oxides of nitrogen (NOx).
Just like any other system, the EGR may fail in many different ways. One of them is failing to reroute enough exhaust gases back into the intake manifold, which triggers the P0401 trouble code.
What makes P0401 code tricky to diagnose, or any EGR system code for that matter, is the different system configurations in use today.
For this reason, you want to have the vehicle repair manual for your particular vehicle on hand. The manual explains the configuration of the EGR system in your car, how it works, and possible strategies to troubleshoot the system when problems arise.
If you don't have this manual yet, you can find a relatively inexpensive copy through Amazon. Haynes manuals include descriptions for the different vehicle systems, parts identification, step-by-step troubleshooting procedures, and how to replace components. You'll also find photos and images to make your project easier. Besides, you can look up the service interval for the different systems, so you keep your vehicle running and in good shape for longer periods of time.
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2. What Causes a P0401 Code?
An EGR system may fail in various ways, depending on the particular system configuration installed on your vehicle and type of failure.
The two most common faults include:
- The accumulation of carbon particles in passages or the valve itself
- A solenoid going bad
But you may also be dealing with a:
- Loose, damaged, or disconnected vacuum hose
- A bad solenoid electrical circuit
- Faulty computer driver to operate a solenoid (very rare)
- Failed EGR system sensor
- Bad vacuum switching valves
If necessary, consult your vehicle repair manual.
3. Common P0401 Code Symptoms
There are symptoms that may point to a restricted or blocked EGR valve or passage.
The two most common symptoms are:
- Detonation (spark knock or ping) as you accelerate or at a steady and moderate speed (cruising). This is a serious symptom since detonations are very harmful to the engine.
- Increase in oxides of nitrogen (NOx) emissions. Your vehicle will fail the emission test because of this.
Other symptoms may include:
- Check engine light (CEL) on, depending on model
- Higher engine temperatures
4. How Does an EGR Valve Work?
The EGR valve remains closed when:
- The engine is at idle.
- The engine is cold or below operating temperature.
- The throttle valve is wide open (to allow the engine to develop extra power as needed).
Other than that, the EGR valve will open gradually to allow exhaust gases back into the combustion chambers as commanded by the car's computer or increased intake pressure, depending on engine model.
5. How to Repair a P0401 Code
In general, repairing a P0401 problem may not be as complicated as it may seem at first. However, some models make it easier than others.
For a successful diagnostic and repair, you need to know the configuration of the particular EGR system installed in your vehicle, how it operates, and how it is monitored.
You'll find this information, and most likely some suggested tests, in your vehicle repair manual.
Two Types of EGR Systems
Broadly speaking, you'll find two types of EGR systems in use today.
1. The old type uses vacuum to operate the EGR valve for the introduction of exhaust gases into the intake manifold.
Within this older system, you'll find ported EGR valves, and valves operated through positive or negative back pressure.
2. Modern vehicles use the car's computer to control the EGR valve through the use of solenoids, vacuum, and/or sensors. Different models vary in their approach.
General Motors, for example, introduced a digital, electronic EGR valve using three solenoids to control valve operation. Also, some models, including some from GM, use a linear EGR valve with a pulse-width modulated solenoid.
Tools to Diagnose an EGR P0401 Code
Because of these two general configurations, diagnosing a P0401 code may require some diagnostic tools:
- A hand-held vacuum pump, for vacuum-operated valves
- A digital multimeter with a minimum of 10 Megohms of impedance to check sensors and electrical circuits
- A scan tool, for electronic-controlled systems
Sometimes you'll need one, two or three of these tools to diagnose your EGR system.
If you don't have the tool you require for the job, check with your local auto parts store. Many of them will loan you one.
Sensors to Keep an Eye On:
Different systems make use of different sensors to determine EGR valve operation. Sometimes, it is necessary to test these sensors as well. On relatively modern and modern vehicles, the car's computer may use a:
- Manifold absolute pressure (MAP) sensor
- Differential (or Delta) pressure feedback EGR (DPFE) sensor
- Oxygen (O2) sensor
Any of these sensors may help the car's computer confirm exhaust flow. And a faulty sensor may trigger a P0401 code.
Knowing how your car's computer monitors the EGR valve or system will help you make a better and more accurate diagnostic.
6. Is a P0401 Code Bad?
Operating your vehicle with a driveability issue is never a good idea. However, there are some car problems that can produce more damage than others when not fixed on time.
A P0401 code is no different. If you were only dealing with an increase in emissions, probably you'll have a bit more time to work on the problem. Although, driving around and polluting the environment because of an engine fault is not a good idea either.
Still, depriving the engine of recirculated exhaust gases means higher engine temperatures and the risk of detonations, depending on how your car's computer is programmed to handle the problem. However, you only need one detonation with enough power to destroy your engine.
So not dealing with a P0401 code on time can leave you with a very expensive repair.
7. How to Clear a P0401 Code
On monitored EGR systems, you can use a scan tool to clear the P0401 code from the computer's memory after making the repair. This will help you verify that you actually have found the problem and fixed it.
The computer will keep monitoring the system as well as the rest of the emission systems. If the computer finds that a signal from the EGR system is out of range, it will set a diagnostic trouble code (DTC).
If the EGR system were to fail in a consecutive trip, the CEL will turn on to let you know there's a problem.
In this case, just download the codes from the computer's memory to see what triggered the CEL.
- How to Replace an EGR Valve
Replace the EGR valve and remove carbon deposits to ensure proper operation of the emission control system and eliminate engine driveability issues.
- How to Test an EGR Valve: A Simple Procedure
Learn how to test an EGR valve in a few minutes to confirm you actually need a new replacement.
- How to Clean an EGR Valve
Do a cleaning EGR valve job at home to improve engine performance and prevent serious engine mechanical problems.
- Symptoms of a Bad EGR Valve
Recognizing symptoms of a bad EGR valve will help you troubleshoot potential problems sooner and restore engine performance faster.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2021 Dan Ferrell