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The crankcase pressure sensor provides the powertrain control module (PCM) with the appropriate crankcase pressure values to maintain a healthy atmosphere inside the engine. If the PCM detects that one or more electrical values are operating outside the crankcase pressure sensor circuit’s desired range, it will set code P051B.

What Does the P051B Code Mean?

Diagnostic trouble code (DTC) P051B stands for “Crankcase Pressure Sensor Circuit Range/Performance.” This is a very rare code that typically only occurs on later model Fords.

The crankcase pressure sensor tells the PCM how much pressure is building up inside the engine. The PCM needs accurate pressure sensor values to prevent seals and gaskets from getting damaged due to excessive pressure. The sensor also monitors pressure while unburnt fumes go back into the engine via the positive crankcase ventilation (PCV) system.

mechanic checking under the car hood for diagnosis
Diagnostic trouble code (DTC) P051B stands for “Crankcase Pressure Sensor Circuit Range/Performance.”

A clogged PCV system might cause crankcase pressure to build up, which could lead to gasket failure and oil leaks. Once the PCM detects incorrect signals from the crankcase pressure sensor, the control module will set code P051B.

You can learn more about the positive crankcase ventilation system in our technical discussion about PCV principles.

Note: The definition of code P051B may be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What are the Possible Causes of the P051B Code?

  • Defective crankcase pressure sensor
  • Electrical issues
  • Defective PCV valve
  • Clogged PCV system and other issues, such as:
    • Broken runners or tubes
    • Disconnected or chafed lines

What are the Common Symptoms of the P051B Code?

check engine light on as a common symptom of P051B
An illuminated check engine light is a common symptom of P051B.

How to Diagnose the P051B Code

P051B is a generic DTC that applies to vehicles equipped with on-board diagnostics (OBD). However, protocols for diagnosing this code and the steps needed for repair might vary depending on the vehicle’s year, make, and model.

If you’re not familiar with the process of diagnosing trouble codes yet, it’s a good idea to bring your vehicle to a licensed mechanic. But if you’re a seasoned DIYer who knows how to work your way around conducting diagnostic tests on your vehicle, then you may go ahead and do it yourself.

To help you out, we’ve rounded up a couple of videos to give you an idea of what the process might involve:

How to Fix the P051B Code

Clearing a P051B trouble code may be a frustrating and confusing task if you don’t have the right tools and automotive know-how. If you’re not confident with your DIY skills yet, it might be a good idea to leave the job to a professional to ensure that the steps for diagnosis and repair are followed correctly.

If you plan on doing the job yourself, make sure you brush up your knowledge by reading the appropriate repair manual or referring to an online repair database for vehicle-specific repair information.

Positive Crankcase Ventilation (PCV) Principles

All engines have blowby (pressure escaping past the piston rings on the compression and power strokes), and without crankcase ventilation, oil seals and gaskets will be compromised by crankcase pressure.

For many years a “draft tube” was used to deal with crankcase pressure, but that tube simply vented crankcase pressure to the atmosphere.

Then in the beginning of the battle against smog in large cities, in the very early 1960s, the Chevy Corvair was equipped with the first positive crankcase ventilation (PCV) valve, which provides a closed system that draws crankcase pressure into the intake manifold. But rather than having the more familiar type of PCV valve, some Asian makes simply connected a vacuum hose to the crankcase drawing air through a small orifice. Jeep Cherokee and Wrangler 4.0L do this, too, as does GM on some of their pickup engines.

The issue with drawing crankcase pressure into the intake manifold is that a filtered vent is needed somewhere else in the crankcase to allow air to enter the crankcase, because a properly operating PCV system will pull more air through the crankcase than the blowby being produced by the piston ring blowby. Thus, if the PCV system is working properly, air will be entering the closure point, whether it be a filter in the breather or a closure hose connected to the air inlet tube between the mass airflow sensor and the throttle body.

So where’s the problem with PCV that it needs this sensor? Well, the PCV system can stop up sometimes because it’s always processing steam composed of water, oil, and fuel, and that clogging can cause crankcase gasses to exit through the closure hose. When that begins to happen, the ECM/PCM detects it with this sensor and stores a code.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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