Diagnostic trouble code P0401 indicates insufficient exhaust gas recirculation (EGR) flow. When measured EGR flow is below normal parameters a code will set. On many vehicles, the MAP sensor measures the change in EGR flow. On most Ford models, the DPFE measures EGR flow.
It's common to see unnecessary EGR valve replacement in an attempt to solve this code. Do not assume that P0401 is simply an EGR valve fault. Sometimes, EGR flow is obstructed by carbon which forms in the intake manifold, and this reduced flow through the EGR passage is what sets code P0401.
G.M. - Code P0401
The photos below shows what an EGR carbon problem might look like on a Chevrolet with a 3.4 liter engine. This view is looking into the throttle body of a Chevrolet Malibu. An EGR passage just beyond the throttle plate is completely plugged and the carbon is easily seen by looking into the throttle body opening. Depending on the engine type and severity of the problem, carbon may not always be this easy to spot.
Removal of the throttle body from the upper plenum allows for better access to reach the carbon stricken EGR passage. A closer look reveals a major build-up of carbon.
Removing the upper plenum from the engine on this model allows for easier cleaning and minimizes the chance of carbon pieces falling into the engine.
The photo below shows the EGR passage after removal of carbon. The area of the EGR tube where cooler fresh air passes is usually where the greatest amount of carbon will form.
Ford - Code P0401
Certain Ford 4.6 and 5.4 liter engines are prone to carbon build-up, but the carbon may not be visible in the same manner as with the G.M. 3.1 or 3.4 liter engine pictured above. Many Ford models also use what's called a DPFE to measure EGR flow. DPFE is an acronym for "Delta Pressure Feedback EGR." Many times a defective DPFE sensor will cause an insufficient EGR flow code. DPFE sensors on many Ford models are know to have a high failure rate. DPFE sensors can be tested with a vacuum source and a voltmeter or scan tool. Be sure to inspect DPFE hoses for leaks and make sure that each hose is on the correct port and be sure all EGR related vacuum hoses are connected properly and leak free.
Chrysler - Code P0401
Chrysler EGR codes are often caused by a defective EGR transducer. Heat damaged diapragms, melted connectors at the solenoid, or water related corrosion are all common problems. Luckily, Chrysler sells most of it's EGR valves as a complete assembly with all related parts; valve, transducer, and hoses. This makes repairing EGR problems on Chrysler products almost enjoyable. Some engines such as the 2.0 liter found in vehicles like the Dodge Neon are prone to carbon blockage in the EGR tube at the intake manifold.
Honda P0401 Codes
Honda may have a recall on some models with carbon blockage. If you own a Honda or Acura with a code P0401, call your local dealer with your VIN number to see if your car (V-6 models) needs the updated intake manifold replaced to resolve this problem.
The YouTube video below shows how one Honda owner solves his P0401 problem on his four cylinder equipped 98 Honda Prelude.
Toyota - Code P0401
Not to say that the EGR valve, modulator, or associated ports and hoses should be overlooked, but, the three words "Vacuum Switching Valve" will describe the fix most often associated with EGR flow codes on a Toyota.
Do not allow deposits to fall into combustion chamber when cleaning carbon accumulation from EGR passages.