P0401 Exhaust Gas Recirculation Flow Insufficient Detected

EGR stands for Exhaust Gas Recirculation and it does pretty much it’s namesake by rerouting a prescribed amount of exhaust gas back into the intake to be drawn into the cylinders to help reduce NOx emissions. Exhaust gas is fairly inert meaning it’s already spent and does not help to create more power or combustion but it does help cool the combustion chamber which helps reduce NOx emissions, NOx emissions are created at temperatures above 2500ºF (1371ºC) and the addition of this inert gas helps bring combustion chamber temperatures down below that threshold.

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During periods of high engine load combustion chamber temperatures can climb well above that number therefore the EGR system is used to help reduce those temperatures and help eliminate the production of NOx emissions in the combustion chamber. NOx is one atom of Nitrogen combined with any number of Oxygen atoms hence the reason they use a ‘x’ after the O because it could be NO1, NO2, NO3…. you get the picture. NOx is one of the main gases responsible for smog and is pretty high on the list of emissions that vehicle manufacturers and CARB (California Air Regulations Board) seek to minimize due to their damaging effects on air quality and the environment. In summary NOx is bad news and the EGR system is one way of helping to eliminate these harmful emissions.

Manufactures use different strategies to help monitor EGR output, one method is to use a pressure sensor of some kind in the EGR stream like a “DPFE” (Differential Pressure Feedback EGR), Ford uses this type of system I believe, which monitors the flow of EGR directly using this sensor, it’s important to note that a failed DPFE sensor can set a P0401 code so keep that in mind if you need to do any testing of the system. The other method of EGR detection is to monitor the O2 sensor output and compare that to an EGR command, Honda does this and is the system I’m most familiar with. On these systems the computer commands the EGR to activate and then monitors the O2 sensor output and looks for a change in reading that would indicated that the EGR is indeed flowing as commanded, if no change is seen or not enough of a change within certain parameters then the P0401 code will set.

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The most common thing people tell me when they come to me with questions about a P0401 is, “I’ve replaced my EGR valve and I still get a P0401″. While it is true that a malfunctioning EGR valve could be responsible for this code, in my experience it’s one of the LAST places to look for the cause of a P0401, remember the code is for ‘insufficient flow’ not necessarily a broken EGR valve, EGR valve failures have their own codes and will normally set if there is a problem with the valve itself. Therefore the first thing I do when confronted with this code is to go in and check the passages or tubes leading to the intake manifold that rout the EGR gases. These passages and tubes often get clogged with hard carbon deposits as well as a smelly ‘gook’, I believe these deposits are the result of the particulate matter, oil fumes, and contaminants from the air itself in the exhaust as well as the condensation that happens when hot exhaust gases pass into the cool intake chamber, this condensation combines with the impurities in the exhaust and ‘drops out’ to become these deposits. For this reason the first thing I do is look to see if there are clogged passages in the EGR system that can be cleaned, most times there is no cost to this outside perhaps a gasket or two so instead of replacing an expensive EGR valve look for clogged passages first. Honda’s are famous for this, both the 4 cylinder F series engines found in many Accords and the J series V6’s used in everything from Accords to Pilots and Odysseys. With the F series there is sometimes a removable plate under the fuel injectors that you can remove to expose the passages and any deposits that may have formed, here is a video that I did for that operation.

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However if you have the older style F series engine there is no removable plate and you need to drill into the EGR inspection plugs in each intake runner and clean them out that way, the next time I have an opportunity to service an EGR system on one of these engines I plan to make a video covering that procedure as there are a few techniques for removing these plugs and sealing the holes after the job is done.

Also on these older engines you may not set a code for EGR flow, instead you might experience a slight miss or hesitation under load or acceleration, this ‘miss’ is caused by only SOME of the EGR passages being clogged therefore some cylinders will get EGR while others do not thus causing the miss so if you have an older Accord 4 cylinder that exhibits this symptom and you’ve already addressed the ignition system perhaps take a look at the EGR passages for blockage as the cause of the problem.

The J series V6 is a little different. Early model versions of this engine had a single passage in the intake for EGR, here is a video I did on cleaning those systems.

Later model J series engines used a removable plate on the top of the intake plenum which contained the passages for EGR to run to each individual cylinder, these are actually much easier to service as all you need to do is remove the plate, clean the holes in the lower part of the intake, clean the holes in the gasket, and then clean out the ‘ant farm’ inside the upper portion of the plate, if you saw what I’m talking about you would get where that name comes from. I often scrape out the bulk of the blockage with a screwdriver and then use throttle cleaner or carburetor cleaner to finish off the rest of the buildup. You CAN reuse the gaskets on the J series intakes as I’ve done it many times with great success however make sure you place the gasket correctly when you reinstall it and if you have a fluctuating idle after the operation look for vacuum leaks, there may be a problem with the gasket OR how the plate or intake is fastened to the engine. I’ve also seen clogged EGR passages on the J series cause misfire codes P0300, P0301, P0302, P0303, P0304, P0305, and P0306 so if you have any of these codes and can’t find a direct cause elsewhere it’s a good idea to check the EGR passages for restrictions.

I honestly don’t have a great deal of experience with the DPFE style of EGR systems however my friends Duane from RealFixesRealFast, ScannerDanner, and Wells Electronics have great videos on the diagnosis of this type of system, here’s some links to their videos on those types of systems.

RealFixesRealFast

Wells Electronics

And lastly these from ScannerDanner

In summary when you get a P0401 start by checking for clogged passages, then check the operation of the EGR and the EGR pressure sensors should there be any. Also remember EGR is mostly active under load so if you experience performance problems during acceleration or pulling up a hill then perhaps the EGR or the EGR flow is at fault, in short don’t replace the EGR valve first thing when you get a P0401 but instead check the flow and the passages involved with your particular system as the first step in your diagnosis If I didn’t touch on it here consult the service manual for your particular vehicle for the proper testing procedure for a P0401 as it can vary greatly from one vehicle to another.

Stay dirty

ETCG

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