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p0401 Information and Advice

p0401 "insufficient EGR flow" means that there is not enough exhaust gasses flowing back into the incoming air to the engine. EGR stands for "exhaust gas recirculation". Car manufacturers created this system so already burned exhaust gas can flow back into the engine (when the EGR valve is open) and lower the combustion temperatures. This helps reduce emissions.

This OBD code basically means that the cars computer has tried to open the EGR valve, but it cannot get enough exhaust gas into the engine. EGR systems used to be controlled by engine vacuum via hoses and valves. Now most of them are controlled by electrical solenoids.

Some Common Causes

EGR Valve
Blocked EGR Passageways
EGR Control (No or Low Vacuum)
Faulty solenoid or wiring

Not As Common Causes

The Cars Computer
DPFE (Delta Pressure Feedback EGR) Valve
Modulator Valve

What To Do

Each car manufacturer has their own EGR system design but the most likely two causes of this code are the valve itself or passageways (many times the intake manifold will have very small passageways that get clogged with soot) that the exhaust gasses pass through.

Many times this code is caused by clogged EGR passages, and sometimes removing the sensor will give you access to the clogged EGR passages.

Another thing that you can do yourself is to check all vacuum hoses and all wiring going to the EGR valve. You might find a hose with a hole in it or you could find a connector that is not seated all the way or some wiring that has been rubbed through.

Final Advice

Code p0401 is a tougher one to diagnose unless you have good experience and some specialized tools. Many times this code will go away if you replace the EGR valve (but definitely not always). You can try just replacing the valve if you do not want to have it diagnosed but diagnosing it is going to be the only way to be sure that you are fixing the problem.