Several things could cause this:
* EGR valve stuck closed: This is the most common culprit. The valve may be physically jammed, clogged with carbon buildup, or have a faulty solenoid preventing it from opening properly.
* EGR valve stuck open: While less common than being stuck closed, a stuck-open valve can also trigger this code.
* Vacuum leaks: The EGR system relies on vacuum to operate. Leaks in the vacuum lines can prevent the valve from functioning correctly.
* Clogged EGR passages: Carbon deposits can build up in the passages where exhaust gases flow, restricting flow and triggering the code.
* DPFE sensor (Differential Pressure Feedback Sensor) malfunction: This sensor measures the pressure difference across the EGR valve and can fail, leading to an incorrect reading and triggering the code. (The 2001 Focus may or may not have a DPFE sensor; check your vehicle's specifications).
* Faulty PCM (Powertrain Control Module): Though less likely, a problem with the PCM itself could cause this code.
To diagnose and fix the problem:
1. Check the EGR valve: Visually inspect it for carbon buildup and try to manually move the valve to check for free movement. Cleaning the valve is often a simple fix if it's just clogged.
2. Inspect vacuum lines: Look for cracks, leaks, or loose connections in the vacuum lines connected to the EGR valve.
3. (If applicable) Check the DPFE sensor: Test its functionality or replace it if necessary. A faulty DPFE sensor is often implicated in P0401 codes.
4. Check EGR passages: This might involve more involved cleaning or even replacing parts of the intake manifold if severely clogged.
5. Consider a professional diagnosis: If you're not comfortable performing these checks yourself, take your Focus to a qualified mechanic for diagnosis and repair.
It's crucial to address this code promptly as continued operation with a malfunctioning EGR system can lead to further engine problems.
