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Diagnostic Trouble Code (DTC) P0401

Exhaust Gas Recirculation Insufficient Flow

Circuit Details

• By any chance if high combustion temperatures are created by oxides of nitrogen then an Exhaust Gas Re-circulation (EGR) system which is the chief feature of this system will be boosted to counter this.
•  This is done by supplying small amounts of exhaust gases back into the combustion chamber.
• Combustion temperatures are minimized as soon as the air/ fuel mixture dilutes the exhaust gases.
• The EGR valve which is linear has been specially designed to generate exhaust gases to the engine without using intake manifold vacuum.
• This valve regulates exhaust flow entering the intake manifold from the exhaust manifold through an orifice with an ECM controlled pintle.
• The pintle position is monitored by the ECM using inputs from the Throttle Position (TP) and the Manifold Absolute Pressure (MAP) sensor.
• This task over, the ECM then directs the EGR valve to operate as necessary by controlling an ignition signal through the ECM.
• The ECM further monitors the results of its command through a feedback signal.
• The required EGR position can be checked with the help of a scan tool.
• By sending a 5 volt reference and a ground to the EGR valve, a voltage signal representing the EGR valve pintle position is sent to the ECM.

• The scan tool can also be used here to note the feedback signal which is the actual position of the EGR pintle.
• The correct EGR position should always be close to the commanded or Desired EGR position.

• This Diagnostic is capable of identifying a reduction in EGR flow.

Diagnostic Tools

• The EGR Decel Filter is an average of the difference in the expected MAP change and the actual MAP change caused by opening the EGR valve during a deceleration.
• It is used to needed when the MIL has glowed.
• To note whether an error is brewing and also to verify its repairs the EGR Decel Filter value renders good help.
•  You can determine if the EGR system is working fine or whether it is partially restricted, or fully restricted if you drive the vehicle up to approximately 97 km/h (60 mph) and then decelerate it to 32 km/h (20 mph).
• If you see a negative number (less than -3) then it implies that the system is working normally.
•  But if you see a positive number then it implies that the system is being restricted and that the desired amount of EGR flow is was not noted.
• In case there is a number that reads between -3 and +2 then it implies that the system is partially restricted but not sufficiently restricted to result in an emissions impact.
• It must be noted that the EGR Decel Filter value should always remain -3 or lower.
• In case you see the EGR Decel Filter number turning more positive (towards 0 or more), then it is deemed that the EGR system is on its way to getting restricted.
• In such a case see if there is any damage likely to be made to the EGR pipe or a restriction initiated by carbon deposits in the EGR passages or on the EGR valve.

Purpose of the Test

Number(s) below refer to the step number(s) on the Diagnostic Table.

1-The On-Board Diagnostic (EOBD) System Check assists the technician to finish some routine checks and store the freeze frame and failure records data on the scan tool if applicable. An electronic copy of the data is obtained when the malfunction occurs and this data is then stored on the scan tool for future use.

2-In this step the EGR valve open is directed to ensure if the EGR system is completely restricted or partly restricted.

3- Here we visually or manually check the EGR passages and valve and see if there are excessive carbon deposits or damage present.

4- Here we ensure that all gasket materials are removed from the EGR mounting surface. A wee amount of material can also cause a DTC P0401 to set.

5-This step confirms whether there is any error and also testifies if a repair solved the problem. If you see that the EGR Deceleration Filter value settles near 0 or a positive number after several tests have been run, then a small restriction may still remain. You must look up the EGR pipe to fish out any damage or dents and also the EGR valve and see if any excessive carbon builds up is there. Only 1 test per ignition cycle should make it perform unless a DTC P0401 has been cleared or the battery de-linked.

6- A very essential step for this diagnostic is to clear the DTC. No sooner it is cleared than the it allows the EGR valve to get a new pintle position since the existing pintle position was flawed because of a failure that initiated the DTC. The DTC needs to be unset keeping the ignition ON, and the engine OFF or when the engine is idling. However the new pintle will not be obtained in case the ECM sees an EGR command.

HOW TO CLEAR A DTC

DTC P0401 - Exhaust Gas Recirculation Insufficient Flow