* Faulty Engine Coolant Temperature Sensor (ECT): The P125 code is straightforward. Even though you've replaced the temperature sensor, it's possible the *new* sensor is faulty, the wiring to the sensor is damaged (short, open, or corroded), or there's a problem with the PCM (Powertrain Control Module) itself. The PCM uses the ECT reading to control various engine functions, including the EGR system. An incorrect temperature reading can cause the PCM to mismanage the EGR system, triggering the P0401 code.
* Underlying EGR System Problem: The P0401 code persists even after replacing the EGR valve solenoid. This suggests a problem *beyond* the solenoid. The EGR system's flow can be insufficient due to several reasons:
* Clogged EGR valve: The valve itself might be sticking or clogged with carbon buildup. Replacing the solenoid doesn't clean the valve.
* Clogged EGR passages: The passages in the intake manifold or exhaust manifold where EGR gases flow can become blocked.
* Vacuum leak in the EGR system: A leak in the vacuum lines connected to the EGR system can prevent proper operation.
* DPFE sensor (Differential Pressure Feedback EGR Sensor): This sensor measures the pressure difference across the EGR valve. A faulty DPFE sensor can lead to inaccurate readings and a P0401 code.
Troubleshooting Steps:
1. Verify the new ECT Sensor: Double-check that you installed the correct sensor and that the connector is securely connected. Use a multimeter to test the sensor's resistance to ensure it's within the manufacturer's specified range.
2. Inspect Wiring: Carefully inspect the wiring harness for the ECT sensor and the EGR system for any damage, corrosion, or loose connections. Pay close attention to the vacuum lines.
3. Check for Vacuum Leaks: Inspect all vacuum lines associated with the EGR system. A hissing sound indicates a leak. Use a vacuum pump and gauge to test the vacuum at different points in the system.
4. Inspect and Clean/Replace the EGR Valve: Remove the EGR valve and thoroughly clean it using a carburetor cleaner or other appropriate solvent. If it's severely clogged or sticking, replace it.
5. Inspect EGR Passages: This often requires removing the intake manifold. Look for any carbon buildup or obstructions in the EGR passages. This is a more advanced repair.
6. Test the DPFE Sensor: This sensor is usually located near the EGR valve. You'll need a multimeter and a vacuum pump to test it. A faulty sensor will need replacement.
7. Scan for Codes Again: After each step, clear the codes and drive the vehicle to see if the codes return. This helps pinpoint the cause.
Important Note: If you're not comfortable performing these repairs yourself, take your Mountaineer to a qualified mechanic. Incorrectly diagnosing and repairing the EGR system can lead to further engine damage. The P0401 and P125 codes, especially when recurring quickly after repairs, suggest a more significant underlying issue needs attention.
