How to Fix P0401 Code EGR Flow Insufficient

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EGR Insuficient Flow

Several factors may cause the P0401 code, including a clogged passage or faulty sensor, but it could also mean that your engine is running at too high an idle speed.

The first thing to do is check for any intake leaks and then clean out the passages of the EGR system with compressed air, starting from one end and moving towards the other until no more dirt comes out of it.

What does the P0404 code mean?

OBD2 scanner showing P0401 trouble code

Error code P0401 means there are not enough gases going into the engine in the resulting air. The EGR returns tiny amounts at once from the exhaust pipe to the engine's combustion chamber to reduce the formation of oxidizing smog nitrogen.

Defective EGR valve, corrosion in the intake manifold, or a leak at the EGR sensor may cause the EGR valve to be stuck open or close, resulting in faulty recirculation flow where the combustion chamber reabsorbs the CO2 at an incorrect time. 

Eventually, this faulty absorption will result in overheating of the EGR valve and the car.


  • Low power
  • Poor fuel economy
  • Rough idle or shaking car
  • Stalling while idle
  • Sudden surges
  • Detectable gasoline fumes
  • Weird knocking sounds on the engine
  • No detected symptoms


  • Faulty EGR valve
  • Clogged or restricted EGR passages
  • Plugged EGR cooler
  • EGR valve stuck open/close
  • Manufacturer-specific issues the DPFE sensor
  • Manufacturer-specific problems with EGR valve
  • Defective catalytic converter
  • Carbon buildup in the EGR temperature sensor
  • Vacuum supply issues
  • Electrical problems with the EGR valve control circuit
  • ECU problems

What is an EGR?

Exhaust Gas Recirculation (EGR) reduces the combustion temperature by reducing the number of exhaust gases from the car. Depending on the vehicle, it may use a vacuum-operated or pressure-operated EGR.

By understanding the respective functions of the parts of the EGR, you can prevent the occurrence of the P0401 code, and therefore reduce the cost for repairs and replacement.

Primary Valve Diaphragm

A vacuum valve of the EGR system opens when pressure is applied to the valve and allows exhaust to enter the intake manifold.

Secondary Diaphragm

This part serves as a regulating medium, regulating the actions of the primary diaphragm. Its principal function is to prevent the EGR valve from opening unless backpressure is sufficient.

Ported Vacuum Switch

The ported vacuum switch contains a wax seal that keeps it in place. This seal expands when the engine heats up and forces the sliding plunger to contact a hole on the intake manifold or thermostat housing. If this doesn't happen, a vacuum can leak from the engine and cause an error code p0401.

Vacuum Control Solenoid

The vacuum control solenoid, another component of the exhaust gas recirculation system, controls the vacuum coming in and out of the EGR valve. It is essential in modulating the voltage supplied to the magnetic coils connected to the plunger of the EGR valve.

How serious is the P0401 code?

A failing valve at EGR valves can create extra ignition before sparking, which can cause internal engine damage affecting the pistons and valves. When the check engine lights are on, the vehicle will fail the Emission Test of the excessive NOx gas.

P0401 Repair Costs

Code P0401 will warn about your electronic gatekeeping system problems with a faulty valve or a bad heating sensor. Total repairs and upgrades are between $150 and $750, including parts and labor. 

Keep an eye out for your vehicle's warning signals. When you have a trouble code, contact the nearest mechanic immediately, especially when you don't know how to do it. 

How to Fix P0401 Code

Once your OBD2 scanner detects a P0401 code, start by checking on the root of the problem, then replace or repair the problematic part. It can mean any of the following:

1. Check blockages

  • To fix these problems, start by checking any possible blockages on your EGR valve, vacuum pump, etc.
  • Place the valve on the vacuum level via scanner or vacuum pump and test out the vacuum from the manifold.
  • If you can't see a reduction, perform a check on the valve, cooler, and tub for soot accumulation.
  • If there is carbon buildup, proceed to clean your EGR valve.

2. Clean your EGR valve

EGR System of a car

  • To clean your EGR valve, remove it from its location, slightly off-center to the back of your car's hood.
  • Turn it upside down and fill the cavity with WD40 cleaner
  • Let it rest for 5 hours to remove all carbon buildup.
  • Wipe off the carbon buildup with a clean rag, then replace the EGR valve.

However, when the valve is too dirty, may have leaks, or is too old, it is best to replace it.

3. Clean your intake manifold

A mechanic removing the intake manifold

Another part of your car that may suffer from carbon buildup is the intake manifold.

  • Take out the intake manifold from your car. You can find it on top of your engine and is responsible for your car's combustion.
  • Spray WD40 on the intake manifold openings, then remove all the crud from the intake manifold with a clean rag.
  • Replace the intake manifold correctly, then drive your car to see if it reset the check engine light.

4. Clean/replace your EGR temperature sensor

EGR Temperature Sensor

Try cleaning your EGR sensor first before replacing it. A new sensor can cost between $150 and $200, while the labor cost to replace it ranges from $30 to $80.

  • Remove the EGR temperature sensor from its location. Depending on your car, you may find it near the exhaust manifold or the EGR valve.
  • Disconnect the battery before removing the EGR temperature sensor to prevent an electrical shock.
  • Clean the sensor with a brush and a clean rug. Do not use harsh chemicals on it.
  • Replace the sensor, then replace the battery connection. If it did not reset the check engine light, replace the sensor with a new one.

5. Replace the EGR control solenoid

EGR control solenoid

The EGR control solenoid is the hardest one to replace on your own because of its size and location. However, with the proper tools and repair manuals, you can do it without professional help. You'll also need protective gloves and safety glasses.

  • Remove the EGR control valve and the battery cable.
  • Disconnect the EGR control solenoid hoses. Check the hoses for leaks. You may need to replace these also.
  • Remove the electrical connector and the retaining fastener with a wrench.
  • Remove the control solenoid.
  • To mount it, reverse the procedure of removing it. Check the fasteners to see that they are snug so they won't wobble when the car is running.

Once you have already fixed the issue on your car, drive it to check if the check engine light is still on. Visit this article to read more on how to reset your check engine light. 

OBD2 Code P0401 FAQ

Can you drive with a bad EGR valve?

Yes, however, you'll have to endure a rougher ride, an annoying knocking or pinging sound on your engine, or your car shaking while idling, not to mention that pesky check engine light on your dashboard. 

However, the worst outcome of driving with a bad EGR valve is failing the emissions test. Depending on the state you're in, sanctions may differ. In Washington, it simply means a trip to the auto repair shop with a detailed report of what's wrong with your car. You can then get a re-test and have your car registration renewed. 

In California, you won't be able to renew your car's registration, and you'll drive at your own risk with your expired tags, which means additional penalties once a traffic enforcer flags you. Your best move is to replace your exhaust system, then get a re-test for new registration. 

How do you clean an EGR valve without removing it?

You can clean the EGR valve without removing it with the proper EGR valve cleaners. However, it usually works on diesel cars only. 

To use it, turn your car on, open the hood, then remove the air intake pipe and spray the tube with the cleaner. Let it rest for 15-30 seconds before spraying again.

Replace the air intake pipe, leave the car idling for a few minutes, then turn it off to allow the cleaner to do its magic in the EGR system. Drive it around to see if it reset the check engine light on your dashboard. 

How does an EGR valve gets clogged?

Whether your EGR valve is pressure-controlled or vacuum-controlled, they are still prone to clogging due to carbon or oil buildup. As a result, the valve may remain open or closed, thus causing the exhaust gas back into the cylinders. 

Can I replace my EGR valve myself?

Yes, you can. Replacing the EGR valve is easy, though getting a new one may be challenging. If shopping online, always check the compatibility of your EGR valve with your car. Most online stores like Amazon have a compatibility checker for your car’s make, year, and model. 


How did this article help you in fixing your OBD2 P0401 code? Leave your comments below, and remember to share this with your friends. If you don’t have an OBD2 scanner yet, check out this review of the best OBD2 scanners for your car.