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The Code Reader: P0401

EGR (exhaust gas recirculation) is an emission control and prevents detonation when working properly. At high combustion temperatures, NOx (oxides of nitrogen) is formed.


The Code Reader takes DTCs, looks at when and why they occur and discusses a diagnostic approach and useful tips to help technicians make an accurate diagnosis.

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Diagnostic Trouble Code: P0401 EGR Insufficient Flow


DTC P0401 sets when the control unit determines the EGR flow is less than expected based on the readings from different monitoring criteria.

No time to read this article? Listen to it instead!


EGR (exhaust gas recirculation) is an emission control and prevents detonation when working properly. At high combustion temperatures, NOx (oxides of nitrogen) is formed. The EGR system directs exhaust gas into the intake to mix it with fresh air. This change in the composition of the intake air reduces the combustion temperatures, ultimately reducing the production of this harmful pollutant that forms smog and acid rain. By lowering the combustion temperature, EGR also prevents detonation.

More to Know:

Multiple different methods are used to monitor EGR operation depending on manufacturer, including EGR temperature, intake manifold pressure, front O2 sensor signal, EGR position sensor reading, spark knock and exhaust back pressure.



In most cases your customer will not notice any driveability or other symptoms. The only thing they notice is the MIL illumination on the dash.


EGR sensor readings can be viewed with a scan tool and in most cases EGR operation can be commanded to begin your diagnosis.

Vacuum-operated EGR valves can be opened with a vacuum pump to check operation and a noticeable RPM change will occur at idle if the flow is sufficient.

Regardless of system, one of the more common causes is carbon buildup causing blocked passageways or restricted EGR valves. If all related sensor readings are present and within normal specifications, disassembly and inspection almost always uncovers a blockage.

Be aware that partially restricted passageways may still allow enough flow for proper operation. On higher mileage vehicles, if a defective sensor or valve is causing a problem, clogged passageways may not be far behind and it is always best to inspect for restriction and quote a cleanout to prevent a reoccurrence of the same code.

Before You Begin:

Always look for manufacturer information and TSBs and since there are many different methods of monitoring EGR operation, it is very important to read the OEM information in regard to troubleshooting. Following the manufacturer’s diagnostic flow chart provides you the quickest results in most cases. Be sure and research the system you are working on and determine the logic behind how it works prior to looking at any data or beginning any diagnostics. TS

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