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P051A – Crankcase Pressure Sensor Circuit (OBD2)

P051A means "Crankcase Pressure Sensor Circuit". The control unit has detected a fault in the electrical circuit of the crankcase pressure sensor: the signal or control current is missing or outside the expected window. The code does not say whether the value is too high or too low, so the wiring and the component itself both have to be checked.

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P051A quick summary

Fault codeP051A
MeaningCrankcase Pressure Sensor Circuit
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityMedium
Can I drive?Yes, for a short time. Check the oil filler cap and dipstick. High crankcase pressure can push out seals and cause oil leaks.
Repair cost€0 – €400

How crankcase pressure is monitored

The crankcase pressure sensor monitors the pressure inside the engine block; it helps detect a faulty crankcase ventilation system, an open oil filler cap or heavy blow-by.

Combustion gases always leak past the piston rings into the crankcase (blow-by). The positive crankcase ventilation (PCV) system draws them back into the intake through an oil separator and pressure control valve. Newer engines monitor this with a crankcase pressure sensor to meet emissions regulations.

P051B means the sensor's signal is out of its expected range or implausible – for example because the crankcase pressure is really too high or too low, or because the sensor or its hose is faulty.

Common real causes are a failed PCV valve or oil separator diaphragm, a loose oil filler cap or dipstick, and a blocked breather hose. High blow-by from worn rings or a damaged turbo can also raise crankcase pressure.

Symptoms of P051A

  • Check Engine light on
  • Whistling noise at idle
  • Oil leaks or oil in the intake
  • Rough idle
  • Increased oil consumption

Most common causes of P051A (most likely first)

Mechanic's Pro Tip

A loose oil filler cap after an oil change is the classic cause of this code – check it before anything else.

  1. 1.Damaged wire or loose, corroded connector at the crankcase pressure sensor

    Most likely

    Green or white corrosion, pushed-back pins and chafed insulation near hot or moving parts are the most common findings.

  2. 2.Faulty crankcase pressure sensor

    An internal break or short in the component itself.

  3. 3.Blown fuse or missing supply voltage

    If several codes appeared at the same time, check the shared fuse, relay or ground point first.

  4. 4.Loose or missing oil filler cap or dipstick

  5. 5.Blocked or cracked breather hose

  6. 6.Excessive blow-by (worn rings, turbo seal failure)

Step-by-step diagnostic guide

  1. 1

    Check cap and dipstick

    Check that the oil filler cap and dipstick are fitted and sealing.

  2. 2

    Check the circuit with a multimeter

    With the ignition off, unplug the crankcase pressure sensor and the control unit connector (or back-probe), then check each wire between them for continuity and for shorts to ground and to battery voltage. Compare the component's resistance or supply voltage with the value in the repair manual.

    Multimeter: Wire continuity: below about 1 Ω. Wire to ground with both connectors unplugged: no continuity. Supply/reference with ignition on: usually 5 V for sensors, battery voltage for actuators.

  3. 3

    Read codes and pressure value

    Read all codes and compare the crankcase pressure at idle and when revving with the specification.

  4. 4

    Inspect the PCV system

    Check the PCV valve or oil separator and breather hoses for tears and blockages.

  5. 5

    Check the sensor

    Check the sensor's reference, ground and signal and the hose to the sensor.

  6. 6

    Check blow-by

    If everything else is fine, measure blow-by or do a compression test to rule out engine wear.

How do I read fault codes? →Where the OBD2 port is, which scanner to use and how to read and clear codes step by step.

Frequently asked questions

Can I keep driving with P051A?

Yes, for a short time. Check the oil filler cap and dipstick. High crankcase pressure can push out seals and cause oil leaks.

How much does it cost to fix P051A?

Typical repair costs are 0 € – 400 €, depending on the cause and the vehicle. The most common fixes are: damaged wire or loose, corroded connector at the crankcase pressure sensor; faulty crankcase pressure sensor; blown fuse or missing supply voltage. Always confirm the cause with the tests above before buying parts.

What causes crankcase pressure codes?

Most often a loose oil filler cap, a failed PCV valve or a blocked breather hose.

Is high crankcase pressure dangerous?

It can push out seals and cause oil leaks, and it may indicate engine wear, so it should be checked.

What is the difference between P051A and similar codes?

P051A belongs to a group of codes for the same component that differ in the type or location of the fault: P051B – Crankcase Pressure Sensor Circuit Range/Performance; P051C – Crankcase Pressure Sensor Circuit Low; P051D – Crankcase Pressure Sensor Circuit High; P053A – Positive Crankcase Ventilation Heater Control Circuit/Open. The tests on this page apply to the whole group; the type of fault tells you where to start.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.