Can someone please help me figure out why i have a po401 diagnstic code? i have already changed the egr valve and egr tube. what can it be? it's killing my gas mileage.

Code 401 is for low EGR air flow. The EGR valve allows exhaust gas to be recirculated back thru the intake and reburned.
The problem could be the EGR valve, the sensor, or a blocked line.

someone removed the egr. or someone replaced the engine with a non egr engine. if the engine was changed you ether need to put an egr system on it or change the computer with one made for an egr exempt engine.

A code P0401 most likely means one or more of the following has happened:

• The DPFE (differential pressure feedback EGR) sensor is faulty and needs to be replaced
• There is a blockage in the EGR (tube), most likely carbon buildup
• The EGR valve is faulty
• The EGR valve may not be opening due to a lack of vaccuum

Possible SolutionsIn fixing this code, it is quite common for people to just replace the EGR valve only to have the OBD code return. The EGR valve is not always the culprit.

• Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's & DPFE voltage. There should be a noticable difference in RPM's with the EGR open
• Clean out the EGR valve and/or tubing to remove deposits
• Check the voltage at the DPFE, compare to specified values (refer to a repair manual for your specific model)
• Replace the DPFE sensor (with a good quality / OEM one)
• Replace the EGR valve

Well, the po401 is saying is that the egr valve has restricted flow. After changing the egr and the tube, something else is causing the restriction. a more thorough diagnosis is recommended. The po420 code sounds like a possible catalyst failure or an o2 sensor failure.

Does the egr vavle open ? You will need to use a scanner to test the dpfe voltage when the egr valve is energized you should read around 3.5 4.00 volts. Also you could remove the vacume line from the egr valve and with the engine running you should see around 100 rpm drop in engine rpm at least this would indicate the egr valve is capable of flowing egr gases through the system to the cylinders.

PO401 Insufficient EGR Flow, valve could be stuck or disconected.

It could very possibly the EGR tubes are clogged or the EGR is not opening or could be DPFE; first i would check the tubes on the DPFE and also clean the EGR tubes and egr it self simple brake clean and a hanger can clean the tubes and spray some in the EGR it also can get gunked up and not move properly.

Hope this help to solve it (remember comment and rated this).

DPFE is a reasonable concern. There is also a very thin tube from the EGR to the DPFE that plugs up with carbon. Check the tube prior to the DPFE.

Check for air leak into system after MAF.

Check for

vacuum leaks on intake manifold,

the air tube between the air filter.

intake manifold vacuum hose.

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clean throttle plate with Berryman carburator cleaner

SEE sample picture HERE
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Check for loose or no connected vacuum hose(s).


Take it for a test drive.


Please rate my answer if it's useful to you.


Cheers,

Mustgo

Clean the EGR air passage tube.

Carbon built up is worst inside this air passage tube.

You will need Seafoam or WD-40 and LOT OF TIME to clean out this tube.

A vacuum will help speed up the cleaning.


Please post more information on the comment window.

Check for

vacuum leaks on intake manifold,

the air tube between the air filter.

intake manifold vacuum hose.

===
clean throttle plate with Berryman carburator cleaner

SEE sample picture HERE
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Also remove carbon built up behind the EGR valve and the EGR passage tube.

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Vacuum leak WILL offset the intended 14.7 to 1 air fuel ratio.

More un-metered air (leaks) will lean out the fuel mixture.

Use small amount of Berryman carb clearer at potential air leak area will help you identify trouble spot.

At idle,just spray and listen for the RPM surge.

The in-rush of carb clearer will increase the RPM on the motor.

Record the problem spot(s) and address it later.

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EGR carbon built up require 2+ hours to clean from start to finish.

You will need a new EGR base gasket before your start.

Locate and remove the EGR valve after the vehicle is cool down.

Carefully record the connector and vacuum line (for older vehicle) location by drawing an easily to follow diagram.

Remove the EGR valve.

Inspect the location of the carbon built-up inside the valve and the EGR passage tube.

SOAK these areas with WD-40 spary.

You will need a small screwdriver,cloth hanger and lot of newspaper and time to get this cleaning done right.

RE-SOAK the carbon and let it sit for over-night will also help.

One the last round,pass a shop vacuum of the EGR valve and the passage pipe.

Install the new EGR gasket and connectors.

Allow the vehicle to warm up outdoor to burn up any remaining WD-40.


Take it for a test drive.


Please rate my answer if it's useful to you.


Cheers,

Mustgo