P0401 – Exhaust gas recirculation (EGR) system -insufficient flow detected

Trouble CodeFault LocationProbable Cause
P0401 Exhaust gas recirculation (EGR) system -insufficient flow detected Hose leak/blockage, basic setting not carried out (if applicable), wiring, EGR valve, EGR solenoid, ECM

What Does P0401 Mean?

A malfunctioning EGR system will cause several codes such as P0400 or P0401-P0409 to be displayed. Often a P040x code is caused by a blockage in the EGR system and is not caused by a failed component.

Possible Problems with the EGR System

  1. Most common problem – carbon blockage where the EGR valve connects to the intake manifold
  2. Leaks in the vacuum lines
  3. EGR solenoid not opening or closing

How does the car’s computer detect a P040x code? A typical OBD-II test sequence is shown below:


For a P0401 (insufficient flow) or P0402 (excessive flow) the PCM will monitor the following components during the test sequence shown above:

  1. EGR valve pintle position – if the EGR valve is electrically controlled
  2. MAP sensor – if the EGR valve is vacuum controlled the PCM will read changes in the MAP sensor output in response to exhaust gas recirculation. The voltage from the MAP should increase when the EGR valve is opened.

EGR Components

EGR System

Above is a diagram of an exhaust gas recirculation system (Ford with DPFE). The exhaust gas recirculation (EGR) system has a few primary components:

  1. EGR Valve – recirculates exhaust gas through the intake manifold.
  2. EGR Solenoid – the “brains” of the EGR system
  3. Vacuum Line – present in vacuum operated systems
  4. EGR Tube – intake for exhaust gases
  5. DPFE – stands for Differential Pressure Feedback for EGR

EGR Valves & Timing

Why recirculate? Recirculation of the exhaust gas is primarily done to prevent NOx emissions. High NOx emissions during driving can indicate an EGR problem. Recirculation reduces combustion temperatures below the reaction temperature of Nitrogen and Oxygen. A secondary benefit of recirculation is improved fuel economy since fuel in vapor form is extremely efficient to burn.

  • Timing is very important for exhaust gas systems
  • Recirculation should not occur at idle. If at idle, the recirculation of exhaust gas can lean out the engine – killing it.
  • Appropriate time to recirculate is when the car is driving and at a certain RPM
  • The amount of exhaust gas recirculated will depend on the RPM. Higher RPMs = higher recirculation
  • Control of timing can be mechanical or electrical

Types of EGR Valves

Vacuum Operated EGR Valve

Vacuum Operated EGR

  • The EGR Valve controls the recirculation of exhaust gases to the intake manifold
  • The EGR valve is normally closed preventing recirculation of exhaust gases to the intake manifold
  • EGR valves have pintles that prevent exhaust gas from entering the manifold.


Electrically operated EGR Valve

Digital EGR

  • Normally “5-wire”
  • Most vehicles made after 2000 have electrically operated EGR valves

Vacuum Line

Some cars vary the vacuum pressure instead of the EGR solenoid duty cycle to control exhaust gas recirculation. These systems have on/off EGR solenoids.

EGR Solenoid

  • The brains of the EGR system. It controls the exhaust gas valve.
  • Normally closed
  • When open the EGR solenoid supplies vacuum from the intake manifold to the EGR valve. Vacuum causes the EGR valve pintle to lift off the valve seat which allows exhaust gases to enter the intake manifold.
  • Disabled during idle and wide open throttle. In GM cars the solenoid is disabled in Park or Neutral.
  • Receives signals from the PCM based on engine load and RPM
  • EGR solenoid often vent manifold vacuum to atmosphere when closed (this is important to know when testing for a leaky solenoid)

Duty cycle – one solenoid control method wherein solenoid voltages are varied by the PCM to change the amount of recirculated exhaust gas


  • Normally used on Fords
  • Stands for Differential Pressure Feedback for EGR
  • Similar to a MAP sensor, the DPFE measure the pressure difference between exhaust gases and atmosphere to determine the flow of the exhaust gas recirculation

EGR Tube

  • In an EGR system there is a pipe which comes off the exhaust manifold.
  • The pipe connects to the EGR valve



The first EGR system tests you should perform are EGR idle tests. Both tests involve leaning the engine at idle by allowing exhaust gas to enter the engine through the EGR system.

  1. Apply vacuum to the EGR valve at idle. This test will determine if there is a blockage in the EGR valve/system. If there is a blockage the engine will NOT die when the vacuum is applied to the EGR valve at idle.
  2. The compete EGR system can be tested at idle by disconnecting the car’s electrical connection from the EGR solenoid and replacing it with an external 12 volts to the EGR solenoid. 12 Volts will activate the EGR solenoid, causing full recirculation of the exhaust gases to the engine. At idle full recirculation of the exhaust gases should kill the engine. Your EGR system is mechanically functioning properly if the engine dies because the solenoid has activated the valve and exhaust gases have entered the engine intake. Even if you are able to kill the engine at idle there can be PCM signal problems. Once you have eliminated mechanical malfunction with the above EGR idle tests, you should test the PCM with a scan tool.

Vacuum Operated EGR Valve Testing

  • EGR valve Vacuum Test
  1. Remove vacuum line from EGR valve
  2. Check if there is vacuum on the vacuum line. Vacuum would indicate the EGR solenoid is stuck open – You should test the EGR solenoid (See EGR solenoid Tests)
  3. Apply external vacuum to EGR valve
  4. The engine should die if the EGR valve is functioning

If the engine does not die, either (1) the EGR valve diaphragm is not opening in which case you will need a new EGR valve or (2) the EGR valve passageways are clogged in which case you can clean the passageways and retest the valve.

  • EGR valve Vacuum Line Test
  1. Disconnect vacuum line from the EGR valve
  2. Disconnect vacuum line from the EGR solenoid
  3. Connect external vacuum to the vacuum line
  4. With the EGR solenoid end of the vacuum line plugged apply vacuum to the vacuum line
  5. Check that the vacuum holds. If there is a leak in the vacuum line the vacuum will not hold.


Electronic EGR valve Testing

Two tests – recommended to remove the EGR valve from the car to perform testing of EGR valve

  • Test main power supply to EGR valve (12 volt)
    • i. Obtain wiring diagram of EGR valve to determine which pins are for power
    • ii. Connect 12 volts to the EGR valve
    • iii. Verify the pintle responds to application of voltage and is in the closed position when no voltage is supplied


  • Test position sensing
    • i. Obtain wiring diagram of EGR valve to determine which pins are for 5 Volt reference and PCM signal
    • ii. Connect 5volt reference to the signal wire with digital voltmeter
    • iii. Set voltmeter to measure resistance
    • iv. Press pintle in with screwdriver or other tool
    • v. Watch voltmeter reading. Resistance should decrease proportional to pintle position difference from its closed position

NOTE: If your EGR valve passes the above two tests but you still have an EGR problem you should test the PCM signals to the EGR valve. With a scan tool you can tell the PCM to open the EGR valve at idle and verify that the engine dies.

EGR Solenoid Testing

The EGR solenoid testing is similar to the EGR valve. You will want to check the vacuum integrity and electrical response of the solenoid.

  • 1. Check Vacuum Integrity
    • Disconnect vacuum lines to EGR solenoid
    • Connect hand held vacuum pump to one of the EGR solenoid ports
    • Close off the other EGR solenoid port
    • Check that the EGR solenoid will hold a vacuum
  • 2. Check for Battery Voltage to Solenoid
    • Connect voltmeter to solenoid electrical connector to measure power supplied
    • Turn ignition on but don’t start engine
    • Voltmeter should read battery voltage (12 volts)
  • 3. Check for PCM control (voltage duty cycle to solenoid)
    • Engine must be running
    • May need scan tool to test

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