P0401 – Is Your Car’s EGR Flowrate Clogged Up?

These days, we’re all quite aware of the enormous amount of emissions that automobiles emit. This is quite clear enough through peering into carbon emissions data, and it’s a sign of the times that we’ve got to do something about it. Hence, why most cars today have a myriad of emissions control devices to somewhat mitigate the consequences (to find out more, check out our guide on what is the catalytic converter). That is until problems like a P0401 error code show up.

Among those emissions control systems is the ‘exhaust gas recirculation’ (or, EGR, for short). It works quite simply by channeling a portion of the tailpipe emissions into the engine to lower its heat, hence controlling the output of toxic gases. But what if that EGR system isn’t working right, or couldn’t pass on the appropriate amount of exhaust fumes back into the engine? What does a P0401 error mean?

P0401 Code

Before we get into anything else, what does a P0401 error code mean? In the world of car diagnostics and its many trouble codes, P0401 is defined as “Insufficient EGR Flow”. In essence, an EGR system in your car works by taking in some of the exhaust fumes emitted as part of the combustion process. Therefore, and through the aid of valves and solenoids, pumps it back into the engine to cool it down.

Now, for the EGR system to work effectively, it requires a set amount (or more specifically, volume) of these exhaust fumes being channeled into the engine at a time. But what if there’s a failure with the EGR system, such as a valve or solenoid getting clogged? In that case, the flow of exhaust fumes into the engine through your car’s ‘exhaust gas recirculation’ will be reduced (or, ‘insufficient’, as it says).


This discrepancy will be logged by your car’s computer brain, its ECU (or ‘engine control unit‘). That’ll then save this error as a trouble code (i.e. P0401), and illuminate the check engine light to make you aware. In most cases, your ECU will have to log this error in two consecutive trips before it alerts you through a blinking check engine light. For the most part, your car’s EGR system is quite dependable.

Unfortunately, issues might crop up every now and then, hence why you’re seeing a P0401 error code as it’s failing to properly recirculate the exhaust fumes. According to some estimates, a P0401 trouble code is among the most common ones noted by car owners. While its failure might seem insignificant in comparison, a faulty or non-functioning EGR system can lead to problems with driving your car.

What Is EGR

Speaking of, let’s take a moment to discuss more what the EGR system does, and why it’s quite an important addition to any car. Your car’s internal combustion process takes in air, fuel, a spark (or ignition), and combines with a bit of compression. With that combustion process, your engine creates power for a car to move. However, it also produces harmful by-products that are emitted into the air.

The deadliest one is Nitrogen Oxide (or, NOx), which can cause serious health issues. To combat NOx, it makes sense to control its emission through the combustion process. This is where an EGR system comes in. You can find it mounted between the exhaust manifold (where exhaust fumes exit out of an engine) and the intake manifold (where fresh air is channeled into the combustion chamber).

An EGR system works by pumping in some of your exhaust fumes back into the intake manifold. Thus, going straight into the engine, and mixing in with the combustion process. This may seem somewhat counterintuitive at first. In practice though, these exhaust fumes are inert, and thus, don’t combust in the engine. As such, and since they can’t burn, your combustion process doesn’t emit as much heat.

With lower heat and pressure, this effectively lowers the creation and emission of NOx gases. An EGR system needs to supply your engine with a constant stream of exhaust fumes for this process to work. It does so through a series of valves and solenoids, which opens up to pass along toxic exhaust fumes into the intake manifold. When this flow sees an odd reduction or restriction, a P0401 code appears.

How Does An EGR System Work?

An EGR system consists of three crucial components:

  • EGR Valve – A sort of butterfly valve or flap that opens, closes, or varies somewhere in between.
  • Actuator Solenoid – An electronic solenoid switch that prompts the EGR valve to open and close.
  • DPFE – Or ‘differential pressure feedback electronic’, which senses the flow rate of exhaust gases.

These three work hand-in-hand to ensure that a precise amount of exhaust gases flow back into your engine to control NOx emissions. By doing so, it changes the chemical structure of the contents within the combustion chamber into something more inert. In other words, it’s not as reactive. As there’s not as much oxygen anymore, this combustion mixture burns more slowly and cools the engine down.

On average, the EGR system could cool down the combustion process by a whopping 150°C. There are also other benefits on top of reducing emissions. In some cars, the EGR is used to improve the potency and efficiency of the combustion process, delivering better performance. Elsewhere (and especially in diesel engines), they help to reduce knocking during idle. But how does the EGR work while driving?

Well, when you’re just starting up, the EGR valve is completely closed off. It then gradually opens up as you’re idling or traveling at lower speeds. At times, the valve might be open by as much as 90% of the way through. During hard acceleration though, the engine needs a lot more oxygen to burn. Your EGR, in this instance, shuts off again to ensure maximal combustion, thus producing more power.

P0401 EGR Insufficient Flow

If the EGR system is showing signs of problems, it might throw up more than just a P0401 diagnostics trouble code, however. These could more accurately point you towards what’s caused the underlying issue to appear. In the case of an insufficient exhaust flow back into the intake manifold, you’ll see a P0401 error code. Otherwise, these other P04xx error codes might appear:

EGR-Related OBD Diagnostics Trouble Code Examples
Codes Definition (And Additional Explainers)
P0400 EGR Flow Malfunction (this is a general flowrate-related code)
P0401 EGR Insufficient Flow Detected
P0402 EGR Excessive Flow Detected (the opposite of P0401, where there’s too much flow)
P0403 EGR Circuit Malfunction (there’s a general electrics-related fault in the EGR system)
P0404 EGR Circuit Range/Performance
P0405 EGR Sensor A Circuit Low (one of the EGR sensors has an overly low resistance)
P0406 EGR Sensor A Circuit High
P0407 EGR Sensor B Circuit Low
P0408 EGR Sensor B Circuit High (the other EGR sensor has an overly high resistance)
P1403 EGR Solenoid Low (there’s an electrical issue with the actuator solenoid)
P1404 EGR System Closed Valve Pintle Error
P1405 EGR Solenoid High
P1406 EGR System Pintle Position Error (here, the ‘pintle’ refers to the plunger mechanism)

What Causes A P0401 Error Code To Appear?

There are several reasons why you might be seeing a P0401 error code, and it explains why the EGR is seeing an abnormal reduction in exhaust flow:


  • Faulty DPFE (differential pressure feedback electronic) sensor, which incorrectly reads the flow rate and volume of exhaust fumes, and thus has to be replaced.
  • As exhaust fumes flow through the EGR tubes and passageways, an excessive carbon build-up might be restricting the flow of gases into the engine. This is the most common cause of EGR-related issues.
  • The EGR valve is faulty and isn’t able to allow the right amount of exhaust fumes to pass through and into the engine. Through a build-up of carbon, it might be stuck fully closed or nearly so.
  • Compromised EGR valve plunger mechanism, which can’t open or close the valves properly. Usually, this is due to carbon build-up, as it locks the plungers into a fully-opened or fully-closed state.
  • In older cars, EGR valves are actuated through vacuum pressure, not electronic solenoids. Any lack of vacuum may easily cause the EGR valve to not open at all.
  • Your catalytic converter might be clogged or restricted, affecting the flow of exhaust gases within the engine and through the EGR system.
  • The EGR temperature sensor might be clogged up or fouled by carbon build-ups. This way, it couldn’t adequately read or detect a temperature change for the opening and closing of the EGR valves.
  • Electrical problems within the EGR valve control circuit. For example, it might be caused by damaged wires, frayed cables, rusty pins, or perhaps loose connections.
  • Glitchy ECU/ECM/PCM (needing a reprogramming or software update), which isn’t able to properly manage the EGR system, such as passing along the right signals to actuate the solenoids.

What Are The Symptoms Of A P0401 Error Code?

If you’re seeing a P0401 error code where there’s insufficient flow of exhaust fumes into the engine, you might experience some symptoms, such as:


  • The check engine light will illuminate, as it’s trying to bring attention to the P0401 error code that it’s logged and stored in its memory.
  • There’s bound to be some pre-ignition knocking or pre-detonation of the combustion process. This is due to the fact that most engines these days are tuned to ignite and combust properly only when the EGR is functioning right. Knocking causes serious internal damage within the engine if not resolved.
  • You’ll notice a noticeable reduction in performance. There’s more hesitation while accelerating, as power sluggishly moves your car along. Otherwise, you might also experience odd engine operations, such as power surging.
  • Your engine is more likely to stall, either during idle, although mostly while driving. If not, you could sense the rough idling and shaky or roughness of the engine.
  • You may be able to hear a pinging or knocking sound emanating from the engine, especially while it’s going at higher speeds or accelerating. This is indicative of that pre-ignition knocking from earlier, as it could be quite detrimental over time.
  • With reduced or affected EGR exhaust flow, your car’s combustion process is no longer as efficient as it used to be. Thus, this leads to increased fuel consumption and will worsen as time passes.
  • A faulty EGR system, such as there being insufficient flow, could lead to a higher tailpipe emissions output. If you’re taking your car for its regular emissions testing, it’ll fail the inspections.
  • The catalytic converters might fail prematurely or require replacements more frequently. As your engine now burns hotter and emits more NOx gases, it easily overwhelms the ability of the catalytic converters to scrub through them.

How Can You Diagnose A P0401 EGR Issue?

If you’re seeing a P0401 EGR trouble code, it’s highly recommended that you have it checked out right away. Failure to do so promptly, and this would cause serious internal damage to the engine. The latter is far costlier to repair and resolve compared to most of the EGR systems. To confirm that you have an issue, such as insufficient exhaust flow in the case of P0401, thorough diagnostics need to be done.

This way, you can properly ascertain what’s going on, find the root problems, as well as identify what has to be performed to fix them. As mentioned, the most common causes of an EGR-related failure like a P0401 error code are either a faulty EGR valve or clogged up (with carbon) tubing and passages. This helps to narrow down the potential repairs or replacements that’ll have to take place.

Before that, here are some ways to diagnose (and possibly fix) a P0401 error code:

Step 1: Take Notes And Make An Inspection

First up, you’ll mostly be concerned about double-checking with your OBD diagnostics tool. Make sure that you’re seeing an EGR-related fault logged on the ECU through your scanner, such as P0401. With that being said, it’s a good idea to clear the codes, and give your car a short and controlled test drive.

If the check engine light comes back and the same error codes reappear, then you have at least been able to identify that there’s a legitimate issue at hand. Once that’s done, pop open the hood (it helps to know how to open Mini Cooper hood), and find the EGR valve. It’s a cylinder-like container or canister that’s connected to the exhaust manifold.

Once you’re poking around there, make a visual inspection, and check these items:

  1. See if the vacuum lines, tubing, and passageways of the EGR system are connected properly between the manifolds and valves. Do you notice any looseness, gaps, or physical damage on the tubes?
  2. Ensure that all electrical connections (i.e. wires and connectors) are seated properly and don’t show any signs of damage. It might help to refer to a diagram of the EGR system to know which wires go where.
  3. Carefully inspect the wiring to make sure that there’s no corrosion, loose connections, frayed cables, or any other issues that might impact electrical input. You may also consider checking if there’s a bad ground connection.

Step 2: Conduct A Vacuum Pressure Test

For this, you’ll need a vacuum gauge, and perhaps a partner to help you out. This will help determine if the vacuum pressure within the EGR is sufficient for regular operation. If not, it might point towards possible vacuum leaks or compromises within the system:

  1. Start by mounting the vacuum gauge at the vacuum supply hose end of the EGR system.
  2. Then, have a colleague start up your car, and rev it up to between 2,000 to 2,500RPM. Keep it steady as you check the vacuum gauge.
  3. Once the engine is up to temperature and is running, there should be at least some vacuum pressure.
  4. If there is a lower-than-normal reading or none at all, it could suggest a few problems. These include a loose hose, blocked solenoid (or vacuum switch in older, vacuum-operated EGRs), or a bad vacuum pump.

Step 3: Inspect The Actuator Solenoid

For this, you’ll still need to rely on that vacuum gauge from earlier. The right diagnosis steps will vary based on what type of EGR you have. With a vacuum-operated EGR, all you need to do is to keep the engine running. For electronic EGRs, the solenoids could be prompted open with your OBD scan tool:

  1. Activate the EGR solenoid (as we’ve detailed just now; either keep the engine running or use an OBD tool to manually force it open).
  2. Now, inspect the vacuum pressure at the end of the vacuum hose.
  3. Once energized, the solenoids (vacuum or electronic) should readily open. If not, then there’s likely an issue with the solenoids or electrical connections.

Step 4: Make Sure The Valves Are Opening And Closing

While you’re here, you could also inspect the valves, to see if it’s stuck open or closed while the EGR is running. In particular, we’ll begin by first inspecting its aforementioned plunger mechanism, to see if it’s properly opening and closing the EGR butterfly valve:

  1. Have your colleague rev the engine up to around 1,500 to 2,000RPM, and keep it steady.
  2. Next, look at the movement of the valve stems-slash-plunger mechanism.
  3. It should move to push the valves open, or pull the valves closed as the EGR’s running.
  4. If there’s vacuum pressure, but the valve stems aren’t moving, then that plunger mechanism is your point of failure.

Step 5: Crack Open The EGR System

First, see if you can get your hands on a handheld vacuum pump. This is only applicable for a vacuum-type EGR system. If you have a more modern electronic EGR unit, your scan tool might have an option to force it to open or close for diagnostic purposes. Either way, we can conduct further testing by:

  1. Applying the vacuum pump directly onto the EGR valves. Or, forcing it to open with your OBD tool.
  2. With you forcing the valves to open, see if there’s any appreciable impact on how your car idles, or if it solves the P0401 error code.
  3. If it makes no difference, the valve itself could be at fault. Alternatively, it might be that the EGR lines and passages are completely clogged up or restricted by carbon build-ups.
  4. Should there be rough idling or stalling during your testing, there might be faults within the electrical and electronic control circuits.
  5. If you have the tools and skills, you might consider removing the EGR valve entirely. This way, you can directly inspect what’s going on inside.
  6. If there’s a lot of sooty carbon build-up, you could give it a clean. Just be careful to not damage the diaphragm or any other sensitive parts of the EGR, such as the sensors.
  7. While you’re at it, look beyond the valves and inspect the EGR passages and tubing, as well. See if there’s any carbon build-up there (especially near the manifolds), and give it a clean, too.

Exhaust Gas Recirculation (EGR) Flow Insufficient Detected – Facts:

  1. The P0401 code is an OBD-II generic code that indicates that the EGR valve is not flowing enough recirculated exhaust gases when it is commanded to open the flow of gases into the intake manifold.
  2. The EGR valve may not open all the way due to low vacuum opening the valve from the EGR control solenoid, or the EGR may not be able to hold vacuum.
  3. The EGR temperature sensor may not register enough of a temperature change when the EGR valve is opened, and EGR passages may be partly blocked, not allowing enough flow of exhaust gases into the intake manifold.
  4. Symptoms of the P0401 code include the Check Engine Light being illuminated, engine ignition ping or knock on acceleration, and failing emissions from excessive NOx in the exhaust tests.
  5. To diagnose the P0401 code, a mechanic will scan codes and document freeze frame data, check vacuum hoses, wiring, and connections to the EGR valve and control solenoid, and check the EGR valve is getting enough engine vacuum.
  6. Common mistakes when diagnosing the P0401 code include failing to check and clean the EGR passages when replacing a defective EGR valve and condemning a good EGR valve when the EGR temperature sensor has excessive carbon buildup on it causing the failure code.
  7. A failed EGR valve can cause the engine to have excessive ignition pre-ignition, leading to internal engine damage to the piston and valves, and the Check Engine Light on will cause the vehicle to fail emission testing from excessive NOx gases.
  8. Repairs to fix the P0401 code may include replacing the leaking or clogged EGR valve, replacing a broken vacuum line to the EGR valve or control solenoid, replacing an EGR temperature sensor, or cleaning carbon out of EGR tubes to intake manifold to clear any blockages.
  9. The EGR temperature sensor tends to get a lot of carbon buildup, causing them to lose sensitivity to the heat from the EGR gases, triggering the P0401 code.
  10. It is crucial to diagnose and fix the P0401 code promptly to avoid potential engine damage and ensure the vehicle passes emissions tests.

Final Thoughts On P0401 EGR Flowrate Problems

That then rounds up our look at what a P0401 error code means, and what it entails for your engine when there’s insufficient flow of exhaust gases through the EGR system. In short, the lackluster flowrate of exhaust fumes through the EGR and into the engine could both impact your performance. And not to mention, adversely affect wear and tear, and causes further internal damage to the engine.

Repairing this issue could cost you anywhere from just $150 if a good cleaning and servicing of your EGR system are needed. Alternatively, it could rise up to $750 if you’re required to replace the valves, solenoids, or other key components. Here’s a brief list of what’s typically done to resolve P0401 or other similar issues within the EGR system:

  • Clean out any carbon build-up within the EGR tubing and passages. Start around the valves, and then onto the passageways leading to the intake manifolds. While cheaper than most, it can be exhaustive and time-consuming.
  • Replace a leaky, broken, or clogged-up EGR valve. This is considered a worst-case scenario, as it’s also the priciest option.
  • Replacing a leaking or damaged vacuum line. In stark contrast, this is a much cheaper repair route.
  • Replace the EGR temperature sensor, which is somewhere in the middle of the price range. If the EGR temperature sensor isn’t badly compromised, you could save a lot by simply cleaning the carbon.

Frequently Asked Questions On P0401

Here are a few popular frequently asked questions concerning an insufficient EGR flow, manifesting in a P0401 error code…

What Is Code P0401

A P0401 diagnostics trouble code is defined as “Insufficient EGR Flow”. In layman’s terms, there’s not enough (an insufficient) flow of exhaust fumes through the EGR system, and back to the engine. This is necessary for your car’s NOx emissions control and reducing the heat within the engine.

How To Fix P0401 Code

In most cases, you can resolve a P0401 error code by giving your EGR system a good cleaning. There’s bound to be a significant amount of carbon build-up in the EGR passages and tubes. Otherwise, you’d be required to replace a broken EGR valve, which is another common cause for P0401 to appear.

P0401 Code After Replacing EGR Valve

If you’re getting a P0401 error code after replacing the EGR valve, there might’ve been a fault during the installation process. An insufficient flow rate, in this scenario, may be attributed to a loose tube or line while you were replacing the valve. Hence, causing a vacuum leak past the EGR valves.

What Damage Can A Faulty EGR Valve Cause

Mainly, driving around with a compromised EGR valve is noticed through poor performance as well as a rough running engine. Deep inside the engine, however, a non-working EGR system may cause pre-ignition knocking. If not fixed soon, this alone could cause serious internal damage inside your motor.

P0401 Ford

On most Ford vehicles, such as on the F150 truck, the common cause of a P0401-related issue is due to a damaged DPFE sensor. It’s wise to diagnose the DPFE sensor and the hoses leading to the DPFE. Make sure that there’s adequate vacuum pressure, and that the hoses are connected properly.

P0401 Toyota

With Toyota vehicles, a P0401 code could be triggered by a faulty VSV (or, ‘vacuum switching valve’). It has been known to fail in some Camry and Corolla models. While you’re checking for any carbon build-up or clogging in the EGR system, replacing the VSV often solves a P0401 problem.

P0401 Honda

P0401 trouble codes appear most frequently on Honda models featuring a V6 engine. The EGR ports on these models could easily clog up, as in the case of certain Odyssey, Pilot, and Accord models. You may experience symptoms such as hesitation while accelerating, not to mention power surges.

Leave a Reply

Your email address will not be published. Required fields are marked *