Toyota DTC P0401

Insufficient EGR Flow

What it means: OBD-II code P0401 represents a problem with your EGR system

Exhaust gas recirculation is referred to as EGR, and it’s an integral part of your Toyota’s emissions system. The EGR is used to reduce the combustion temperature and pressure to limit oxides of nitrogen. There are three parts to the EGR system: the valve, the actuator solenoid, and the differential pressure sensor EGR, or DPFE.

Toyota DTC P0401
A clogged ERG valve via Flickr.

These elements work together for delivery of the accurate amount of recirculation based on the engine’s temperature and load. The code P0401 is triggered when there is an insufficient amount of EGR happening.

Possible causes of this problem include:

  • Faulty DPFE sensor that needs to be replaced
  • Blocked EGR tube
  • EGR valve is faulty
  • A lack of vacuum is keeping the EGR valve from opening

To fix the problem and get rid of the code, most people just replace the EGR, and the code comes right back. That’s because the EGR valve is not necessarily the culprit. If no visual cues to the problem are present, use a vacuum pump to open the EGR valve and monitor the RPMs and DPFE voltage. There should be a difference in RPMs when the EGR valve is open. Also, clean out the EGR to clear any clogs or debris that might be causing the valve to stick. The next step would be replacing the DPFE sensor, and after that the EGR valve.

While every effort has been made to provide accurate information about Toyota diagnostic trouble code P0401, this is not meant to be a definitive guide on the subject. This write-up is simply meant to explain the possible causes of the code. If you’re experiencing code P0401, please consult a professional mechanic for more extensive trouble shooting.