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What does Code P0401 mean?

PO401 : Insufficient EGR Flow.

The Exhaust Gas Re circulation (EGR) system is designed to reintroduce exhaust gas into the combustion chambers, thereby lowering combustion temperatures and reducing the formation of Oxides of Nitrogen (NOx).

 

PO401 : Insufficient EGR Flow

The Exhaust Gas Re circulation (EGR) system is designed to reintroduce exhaust gas into the combustion chambers, thereby lowering combustion temperatures and reducing the formation of Oxides of Nitrogen (NOx).

The amount of exhaust gas that is reintroduced into the combustion cycle is determined by several factors, such as: engine speed, engine vacuum, exhaust system back pressure, coolant temperature, throttle position. All EGR valves are vacuum operated. The EGR vacuum diagram for your particular vehicle is displayed on the Vehicle Emission Control Information (VECI) label.

The EGR system is Differential Pressure Feedback EGR (DPFE) system, controlled by the Powertrain Control Module (PCM) and composed of the following components: DPFE sensor (also referred to as the back pressure transducer), EGR Vacuum Regulator (EVR) solenoid, EGR valve, and assorted hoses. The P0401 code means that OBD detected an insufficient amount of EGR.

Symptoms

There may be drive-ability issues such as pinging ( pre-ignition knock , similar to cheap gas) when the vehicles engine is under heavy load or the vehicle is climbing a steep grade. There may also be other symptoms.

Causes

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

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

Possible Solutions

In 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 cause. If you replace all 3 components the like is all but guaranteed to go away . Or you can test each component .

  • Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's. There should be a noticeable difference in RPM's with the EGR open
  • Clean out the EGR valve to remove deposits
  • Replace the DPF sensor
  • Replace the EGR valve

 

COMPONENT TESTING

DPFE Sensor
  1. Disconnect the pressure hoses at the DPFE sensor.
  2. Connect a hand vacuum pump to the downstream pickup marked REF on the sensor.
  3. Using a multi meter, back probe the SIG RTN circuit at the DPFE connector.
  4. With the ignition ON, signal voltage should be 0.20–0.70 volts.
  5. Apply 8–9 in. Hg of vacuum to the sensor. Voltage should be greater than 4 volts.
  6. Quickly release the vacuum from the sensor. Voltage should drop to less than 1 volt in 3 seconds.
  7. If the sensor does not respond as specified, check the power and ground circuits.
  8. If power and ground circuits are functional, the sensor is faulty.
EGR Valve Control Solenoid
  1. Remove the EVR solenoid.
  2. Attempt to lightly blow air into the EVR solenoid.
    1. If air blows through the solenoid, replace the solenoid with a new one.
    2. If air does not pass freely through the solenoid, continue with the test.
  3. Apply battery voltage (approximately 12 volts) and a ground to the EVR solenoid electrical terminals. Attempt to lightly blow air, once again, through the solenoid.
    1. If air does not pass through the solenoid, replace the solenoid with a new one.
    2. If air does not flow through the solenoid, the solenoid is OK.
  4. If the solenoid is functional but the problem still exists, check the power and ground circuits.
EGR Valve
  1. Install a tachometer on the engine, following the manufacturer's instructions.
  2. Detach the engine wiring harness connector from the Idle Air Control (IAC) solenoid.
  3. Disconnect and plug the vacuum supply hose from the EGR valve.
  4. Start the engine, then apply the parking brake, block the rear wheels and position the transmission in Neutral.
  5. Observe and note the idle speed.

  1. NOTE: If the engine will not idle with the IAC solenoid disconnected, provide an air bypass to the engine by slightly opening the throttle plate or by creating an intake vacuum leak. Do not allow the idle speed to exceed typical idle rpm.

  2. Using a hand-held vacuum pump, slowly apply 5–10 in. Hg (17–34 kPa) of vacuum to the EGR valve nipple.
    1. If the idle speed drops more than 100 rpm with the vacuum applied and returns to normal after the vacuum is removed, the EGR valve is OK.
    2. If the idle speed does not drop more than 100 rpm with the vacuum applied and return to normal after the vacuum is removed, inspect the EGR valve for a blockage; clean it if a blockage is found. Replace the EGR valve if no blockage is found, or if cleaning the valve does not remedy the malfunction.