P0401 OBD-II Trouble Code: EGR Flow Malfunction

Car manufacturers and petrolheads hate to agree that automobiles are one of the major sources of pollution that are destroying our environment. But, it is nothing but the truth. That’s why car manufacturers developed some systems to reduce harmful gas emissions. And EGR valve is one of these systems.

The EGR valve can fail in several ways. If your car computer logs error code P0401, it means the computer has detected insufficient flow in the exhaust gas recirculation valve. In this article, I’ll explain the causes, symptoms, severity, diagnosis, and how to address diagnostic trouble code P0401 on Mercedes and other car models. But first, let’s see the meaning of a P0401 fault code.

p0401 toyota

What does error code P0401 mean?

The P0401 fault code stands for Exhaust Gas Recirculation Flow Insufficient Detected. Whenever the powertrain control module (PCM) logs error code P0401 on its memory, it means it has detected insufficient exhaust gas recirculation flow.

To better understand this, you must know what the EGR valve does. The EGR valve is designed to filter a small amount of exhaust gases and send it back into the engine for a re-burn. The primary purpose of the re-burn is to reduce the overall emission from the tailpipe.

However, the EGR valve is subject to failure like other mechanical components in cars. When the failure causes the EGR to send insufficient exhaust gas recirculation into the engine, the car computer will log error code P0401 on Toyota or that respective car model. The PCM may also log related codes like P0402, P0403, P0404, P0405, P0406, P0407, P0408, and P0409 error codes.

What is the symptom of error code P0401?

The signs of fault code P0401 differ on a case basis. This means that P0401 VW and P0401 Honda will have different symptoms. Even so, P0401 signs on a Suzuki Grand Vitara may be different from another Suzuki Grand Vitara. However, some symptoms are more prevalent than others.

Illumination of the check engine light

A more prevalent sign of a P0401 code is the illumination of that little panic-inducing light on the dashboard. Whenever the EGR valve malfunctions, the car computer will project the check engine light on the dash, alerting the driver of an underlying issue in the engine. 

But it’s essential to note that the car computer can trigger an engine warning light for several reasons. So, you have to diagnose your vehicle and address the issue whenever the check engine light appears. If not addressed on time, it could cause something serious that could lead to engine breakdown.

Loss of power

As explained, the EGR valve helps reduce exhaust emission by recirculating exhaust gas into the combustion chamber for a re-burn. The EGR also lowers the combustion temperature by sending cooled exhaust gas into the combustion chamber. If the EGR is not working as it should, it’ll cause issues like sluggish acceleration, difficulty maintaining speed, or loss of engine power.

Failed emission test

The purpose of the EGR is to reduce exhaust emissions. If it stops working correctly, it’ll cause increased emissions into the atmosphere. Of course, this will cause the vehicle to fail an emission test.

Slow acceleration

If the EGR valve is stuck closed or open, it’ll lead to slow acceleration or engine stalling when decelerating and accelerating. This can be dangerous, especially when overtaking a vehicle on the highway.

Rough idling

When the EGR valve is stuck closed or open, it can lead to rough engine idling. You will feel the vehicle vibrating or shaking, especially when idling. Like the check engine light, several factors can cause rough idling. So, don’t point accusing fingers at the EGR valve without diagnosing the vehicle to pinpoint the root cause of the problem.

Ignition pinging

Ignition pinging is a pre-combustion situation where the combustion ignites prematurely. If the EGR valve is bad or you have improper exhaust recirculation flow, it’ll cause ignition pinging. This can happen for several other reasons, like low octane level in fuel, over-advanced ignition timing, lean fuel mixture, and carbon deposits on top of the pistons.

p0401 egr flow insufficient

What Causes error code P0401?

Several factors can cause the ECM to log error code P0401. Most of these factors are prevalent in several vehicles, regardless of the car’s make and model. Here are the causes of P0401 EGR flow insufficient you should check when diagnosing the root cause of the problem.

  • Poor or lack of EGR vacuum due to carbon deposits
  • Obstructed or clogged EGR tube
  • EGR temp sensor is fouled with carbon deposit
  • The EGR is not signaling enough temperature change when the valve is opened.

How serious is error code P0401?

A P0401 diagnostic trouble code is a serious issue and should be addressed immediately. Because a P0401 means increased exhaust gas emissions due to insufficient gas recirculation flow, there will be harmful exhaust gas output.

A failed EGR valve will cause excessive ignition pinging, leading to internal damage to the valves and pistons. Plus, the check engine light will display on the dashboard. As a result, you won’t know when the system registers a more serious issue and triggers the engine warning light because you’ll think it is still the former light.

How to diagnose

A few factors can cause the car computer to log error code P0401. If you want to diagnose and resolve the issue at home without throwing money at parts, follow this guide religiously. It is essential to note that this is an intermediate repair, and it’s not for amateur DIYers. The diagnosing and repair procedures require technical skills more than what a scan tool can offer, and it can be labor-intensive.

Tools and Items Needed

  • Diagnostic trouble code reader
  • WD40 or throttle body cleaner
  • Speedometer cable
  • Socket sets and ratchet

Step 1: Remove & Clean the EGR valve

Get your respective car service manual and locate the EGR valve on your car. Unplug the electrical connector on the EGR valve and loosen the bolts holding it in place. If you find any carbon blockage, use a WD40 or throttle body cleaner and a speedometer cable to clean the blockage.

Spray in throttle body cleaner in the EGR valve and let it sit for half an hour. After that, apply slight gum on the gasket and reinstall the EGR valve. Watch this video for visual clarification. Test the vehicle for 20 to 30 minutes to watch the check engine light disappear. If the light doesn’t go away on its own, use the diagnostic scan tool to clean the light and the logged error code.

Step 2: Check the EGR temperature sensor

Now locate the EGR temperature sensor using your vehicle service manual and check for carbon build-up that may be causing insufficient flow reading. Clean any carbon build-up and reinstall the sensor.

Step 3: Check the wiring connectors and hoses

Inspect the wires, wiring connectors, and hoses connected to the EGR valve and see if there is any crack, disconnection, or damage. Replace any damaged components and erase the registered error code with a scan tool.

Step 4: Clean the intake manifold

Locate and remove the intake manifold. Check for carbon or oil deposits and clean it with WD40 or throttle body cleaner. Before you can successfully remove the intake manifold, you’ll have to remove several components, like the throttle body, the air filter housing, and several connectors and hoses. Watch this video for a visual presentation.

After cleaning the intake manifold, apply slight gum on the gasket and reinstall it following the reverse procedure. If none of these steps could resolve the problem, take the vehicle to a mechanic for a thorough diagnosis and solution.

Common P0401 diagnosis mistakes

One of the common mistakes among DIYers and auto mechanics is failing to clean the EGR passage tubes when replacing a defective exhaust gas recirculation valve.

Another common mistake is replacing a good EGR valve because the car computer logs error code P0401 due to carbon build-up on the valve. In such cases, cleaning the EGR valve is all you need to resolve the underlying problem.

Another notable mistake is failing to see if the EGR control solenoid will hold vacuum before replacing the valve.

How do you fix P0401?

Since a few factors can cause the powertrain control module to throw error code P0401 on Jeep and other car models, there are a few ways to address the issue. You’ll need one or more of the following to address the logged error code.

  • Replace clogged or leaking EGR valve
  • Clean carbon on the EGR temperature sensor if it’s not registering temperature change.
  • Replace the EGR temperature sensor if cleaning it does not solve the underlying problem.
  • Replace a broken EGR valve or control solenoid vacuum line
  • Cleaning the intake manifold with WD40
  • Cleaning carbon deposits on the EGR passage lines.

Approx. Repair Cost

It is impossible to give an accurate repair estimate for a P0401 trouble code without a thorough diagnosis since more than two issues can cause it.

If you want a professional mechanic to diagnose and resolve the issue, most technicians will charge you for an hour of diagnosis. The diagnosis will enable them to find the leading cause of the problem and give accurate repair costs. Most mechanics charge $70 to $150 per hour.

A P0401 on Isuzu and other car models requires fixing one or more of the following. Each potential repair below includes the part fee and the labor charge.

  • Vacuum leak $70 to $130
  • EGR valve replacement $300 to $400.


Can I drive with a P0401 code?

A P0401 trouble code can cause premature ignition and pinging, which could lead to severe damage to the pistons and valves. It is, therefore, considered a serious issue, and you should not ignore it for any reason. Ignoring it for an extended period may cause catastrophe on the internal engine components like the pistons and valves.

Will the P0401 code clear itself?

The answer is yes and no. Yes, because some car models will automatically clear logged trouble codes, and No because other car models will require you to clear the code manually. In the case of a P0401 fault code, some cars will automatically erase the code after you fix the actual cause, while other vehicles will require you to manually erase the code after fixing the problem.

If you wonder if the code will disappear without fixing the problem, the answer is no. You have to fix the problem that triggered the code.

Will a faulty EGR damage the engine?

A faulty EGR can cause several issues, such as insufficient exhaust gas recirculation flow, leading to reduced engine performance, engine stalling, rough idling, decreased fuel efficiency, poor acceleration, and premature ignition. If premature ignition or pinging occurs, it’ll cause damage to the valves and pistons. This will invariably damage the engine.

Can you unclog an EGR valve?

You can easily unclog an EGR valve using a throttle body cleaner or WD40 and a cleaning brush. Locate and remove the EGR valve and spray throttle body cleaner into both holes. Insert the cleaning brush inside the holes one after the other and clean the holes. Repeat this process until the entire carbon is removed. Wipe the surface with a clean towel or rag and reinstall it.

Final Words

If you are a regular car owner like me, you’ll agree nothing is more desirable on the road than having a smooth and seamless ride. Spirited drivers appreciate the glory of cars if the vehicle can seamlessly convert into road beasts on the highways.

A P0401 can cause loss of power, poor acceleration, and rough idling, depriving cars of their glory. The error code is logged when carbon build-up in the EGR valve interferes with the exhaust gas recirculation flow. In most cases, all you need to rectify the problem is to clean the EGR valve. Follow the instructions above to clean and erase the P0401 error code and enjoy a seamless driving experience.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

Recent Posts