This generic powertrain diagnostic trouble code (DTC) typically applies to many OBD-II vehicles. That may include but is not limited to vehicles from Ford, Dodge, Ram, Volvo, etc.

If you have a code P051F stored in your OBD-II vehicle, it means that the powertrain control module (PCM) has detected a signal from the crankcase pressure sensor that indicates an over pressure condition exists.

Air density (pressure) in the engine crankcase is monitored by the PCM using an input voltage signal from the crankcase pressure sensor. Crankcase pressure sensor input voltage is received (by the PCM) as actual units of pressure. Either kilopascal (kPa) units or inches of mercury (Hg) are used to measure crankcase pressure. The crankcase pressure sensor is typically located in or near one of the valve covers.

Because the lower end of the internal combustion engine must be sealed to prevent oil leaks, a certain amount of pressure is created there. This pressure is caused by drastic changes in temperature, the vapors created by engine oil, and the rapid centrifugal motion of the crankshaft, connecting rods, etc.

The positive crankcase ventilation (PCV) system uses carefully controlled intake vacuum to draw pressure from the crankcase through a specially designed valve (PCV valve) that only allows a single direction of flow. In the vast majority of vehicle applications, the PCV valve itself is responsible for filtering unwanted oil and debris and preventing it from entering the intake manifold. In this particular application, a PCV filter has been added as a precautionary measure for protecting the engine air delivery system from excess oil and debris.

If the PCM detects a level of pressure from the crankcase pressure sensor that exceeds maximum parameters, a code P051F will be stored and a malfunction indicator lamp (MIL) may be illuminated.

Photo of a PCV valve, one component of the system:

Excessive crankcase pressure may result in engine oil leaks. A code P051F should be categorized as severe and addressed accordingly.

Symptoms of a P051F engine code may include:

Causes for this code may include:

Photo example of a crankcase pressure sensor:

In my experience, a manual vacuum pressure gauge, a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information will be necessary when diagnosing a code P051F.

A manual vacuum pressure test should be performed prior to diagnosing any excessive crankcase pressure/PCV related codes. If the engine fails to produce enough vacuum, it must be repaired before moving forward with your diagnosis. To conduct a manual vacuum pressure test, disconnect the PCV vacuum hose and attach your vacuum pressure gauge there. Your vehicle information resource should yield specifications regarding minimum engine vacuum.

Check all PCV hoses for signs of cracking or breakage and make repairs as necessary. Cracked or collapsed PCV vacuum hoses may contribute to the conditions which caused the P051F to be stored. If there is sufficient vacuum on the inlet side of the PCV filter but not on the outlet side, suspect that the filter is clogged.

If the engine is in good working order and there are no vacuum leaks, continue with a visual inspection of all crankcase pressure sensor wiring and connectors. Make repairs as needed.

Next, I would connect the scanner to the vehicle diagnostic port and retrieve all stored codes and freeze frame data. Writing this information down may aid you as your diagnosis unfolds. After that, clear the codes and test-drive the vehicle to see if the code is reset.

Utilize your vehicle information resource to obtain connector face views, wiring diagrams, connector pin-out charts, component testing procedures and specifications. All this information will be required to proceed with your diagnosis.

Test the crankshaft pressure sensor using the DVOM

Use the DVOM to test for reference voltage (typically 5-volts) and a ground at the crankcase pressure sensor connector

Test crankcase pressure sensor signal circuit voltage using the DVOM

Test the crankcase pressure sensor signal circuit at the PCM connector

If the crankcase pressure sensor and all circuits are within specifications, suspect PCM failure or a PCM programing error.