Do you know how to fix a P0401 code on Mitsubishi? this is one of the questions our readers ask a lot. Well, we´ve got you covered.
Sometimes it can happen to you that the check engine light comes on your Mitsubishi’s dashboard, while you occasionally hear a knock from under your hood and your engine is slightly sluggish during longer drives.
This is when you might grab an OBD-II reader, and find out that you have a P0401 code.
So, how to fix a P0401 Code on Mitsubishi vehicles? To fix the P0401 code on Mitsubishi, you just need to follow the below steps:
- Step #1 Check EGR tube
- Step #2 Check EGR valve
- Step #3 Check solenoid harness/vacuum tube and box
- Step #4 Check battery
- Step #5 Check DPFE sensor
Table of Contents
Meaning: What is the P0401 Code on Mitsubishi
The P0401 code is one of the many standard OBD codes that you could very likely encounter on your Mitsubishi, especially on older ones.
In the simplest terms, it means insufficient flow through the EGR valve.
EGR or Emission Gas Recirculation valve is one of the devices on your engine that helps decrease the car’s emissions.
It allows for some of the exhaust gases to return back into the cylinders of your engine during the compression cycle.
The effect of this is decreased temperature in cylinders and thus lower production of nitro oxides, NOx.
Because the flow rate can be predetermined based on the RPM of your engine, a sensor can measure it whether it is sufficient or not.
If for any reason the flow is not sufficient, the error code P0401 will be stored.
The flow through the EGR valve can become insufficient for a proper decrease of the temperature inside cylinders by several causes.
Causes of the P0401 Code
The most common cause of the P0401 code hides in the nature and content of the exhaust gases, soot.
It is a simple fact of life and something that is certain to happen with almost every Mitsubishi at some point.
As the exhaust gases go through the EGR tube and valve some of the particulates get deposited in their insides.
So, there is a carbon buildup that can become considerable over time and start obstructing the flow of gases.
This carbon build-up can either prevent the valve from opening/closing or obstruct the flow of gases.
Another potential cause is the inability of the valve to open because of the issues with actuation.
On most Mitsubishi engines the valve is actuated by a vacuum, so leaky vacuum boxes or vacuum hoses can be the culprit.
On Mitsubishi engines, the EGR valve is actuated by a solenoid which supplies the needed amount of vacuum, so its harness could be the issue or battery and its wiring.
Of course, as with any code that is reported by a sensor, there is a chance that everything works fine except the sensor itself.
Diagnosis: Reading P0401 Code
Reading the P0401 is a relatively simple thing to do, all you need is an OBD-II scanner that you need to connect to the port located under your dash.
Depending on its software interface things can be slightly different between various scanners, but almost all of them have some sort of “Scan Codes” button either in software or on them.
You can even use those cheap Bluetooth OBD-II scanners that connect to your mobile phone.
Once you scan for codes, you will get results, either just the P0401 code or also some other codes besides it.
If you have a turbocharged Mitsubishi engine, you will very likely see an error code related to the boost pressure, but don’t worry, it is caused by the P0401 situation and will clear.
Troubleshooting P0401 Code on Mitsubishi
Troubleshooting the P0401 code comes down to checking each potential cause one by one, going from the most likely and easiest to check.
Step #1 Check EGR tube: The very first thing you should do is check the EGR tube for carbon buildup.
It’s easy to recognize the place where it enters the EGR valve or intake manifold under the engine cover.
If there is carbon buildup you should clean it, carburetor cleaners and similar products are good enough.
Part of checking the EGR tube is also checking whether the EGR port on the intake manifold is clogged, which is done visually.
Step #2 Check EGR valve: Nest step is to check the valve itself, especially if the EGR tube is clogged as very likely the valve will be itself too.
But you should also check how well it holds the vacuum with a vacuum pump, after cleaning it.
You have to connect it to the EGR valve’s vacuum port and let the engine idle until it warms up.
Then you should pump the vacuum, if everything is OK the engine RMP will drop by a lot if not even stall and the vacuum will hold.
If there is no change in RPM, the valve is not opening, and if the vacuum doesn’t hold, the valve’s diaphragm is very likely punctured.
Step #3 Check solenoid harness/vacuum tube and box: To check the solenoid you will need a multimeter.
When you disconnect the solenoid 6 pin connector you will need to measure the resistance between pins.
If you count the pins from the top left corner going clockwise, you need to measure resistance between combinations of pins 1-2, 2-3, 4-5, and 5-6.
All these should read the same value between 20 and 25 ohms, if any reads 0, the problem is a bad solenoid.
Because the engine shakes when it works, there is a small chance of the solenoid connector becoming loose, so this is something that you have to check or keep in mind as a potential cause.
Another reason the EGR valve is not opening can be a faulty rubber vacuum line, which you should inspect visually for obvious cracks.
Besides it, the inability of a line to hold a vacuum can be down to a cracked vacuum box, or old gaskets on it.
On some Mitsubishi vehicles, it is inside the front fender or bumper and gets easily cracked in fender benders.
Step #4 Check battery: Because the EGR solenoid is electrically operated, a bad battery can be a cause of it not working properly, which can lead to the P0401 code.
Step #5 Check DPFE sensor: A DPFE or Differential Pressure Feedback sensor measures the flow of recirculated gases.
But over time, it will start reporting sensing lower than actual pressure/flow and eventually report insufficient and store code P0401 in the ECM.
You need a voltmeter to check whether it is good or not, by measuring the voltage on the signal wire.
The signal wire is either top or left one of the three wires it has, and depending on the exact engine model, the voltage needs to be in the 0.45-0.55 or 0.8-1.0 volts.
If it is out of the proper range, it needs to be replaced.
P0401 EGR troubleshooting diagnostic and repair >> Check out the video below:
Prevent P0401 Code
Because of the nature of the exhaust gases, the EGR system is bound to get clogged at some point or the DPFE sensor to fail.
It is something that simply will happen with time.
What you need to do when you are fixing the P0401 code is to fix it for good, as the persistence of this code is a very common complaint.
It happens almost always due to one of two reasons, either mechanic or the person who is fixing it doesn’t clean the ERG port on the intake manifold, or doesn’t check the vacuum box for leaks.
Repair Cost of P0401
The repair cost of P0401 on a Mitsubishi can range between $150, in case of a disconnected vacuum hose, and $750 if replacement of the EGR valve is needed.
Just diagnosis of the P0401 can take a couple of hours, to exclude all possible causes, while EGR valves cost above $200.
The P0401 is a code that means that EGR flow is low.
This system of exhaust gas recirculation serves to decrease the emissions of your Mitsubishi, and in case of this code, your vehicle will fail the emission tests.
Besides running dirty, on turbocharged vehicles, this situation will increase the wear on the turbocharger and so lead to more expensive problems unless fixed ASAP.
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