GMC Sierra P0521: Oil Pressure Sensor Switch → Performance/Range

P0521 An OBD2 diagnostic code is generated when the vehicle is detected as having an OBD2. GMC Sierra It detects that an oil pressure sensor value is not within its normal range.

This is usually caused by an oil pressure sensor problem or an oil system issue. It should be immediately diagnosed and repaired.  It is possible, and even probable that an oil pressure sensor/sensor is defective. However, it could also indicate a problem in the oil pressure.  

Your Sierra’s oil pressure sensor measures the oil pressure mechanically, and converts that reading into a voltage signal that the PCM (Powertrain Control Module) converts into an oil pressure reading.  The oil warning light is activated when this reading appears on the oil pressure gauge.

P0521 GMC Sierra



Índice de Contenido
  1. Quick facts P0521
  2. P0521 Symptoms:  GMC Sierra
  3. Sierra P0521 Causes
  4. GMC Sierra Diagnosis P0521
    1. 1. Take a look at the Oil
    2. 2. Examine the Wiring Harness
    3. 3. Manually check your oil pressure
  5. Conclusion

Quick facts P0521

  • Most often caused by an issue with your Sierra’s oil pressure sensor or circuit
  • You should take P0521 very seriously
  • Most common symptoms are the oil light and an incorrect oil pressure gauge reading.
  • Simple to diagnose


P0521 Symptoms:  GMC Sierra

Below are the top symptoms of P0521.

  • Glow-/Low Oil Light The most common cause of P0521 is a poor oil pressure sensor. However, the oil pressure gauge readings and the low oil warning should be treated as accurate until you are certain.
  • Ticking or Knocking - If your engine is ticking and knocking, it’s likely that the oil pressure sensor is accurate and there’s a problem with the engine or you need oil.
  • Check Engine Light– That’s why you are here.


GMC Sierra P0521 Diagnosis


Sierra P0521 Causes

The problem with your sensor wire or sensor sensor is what causes P0521. This would indicate that your engine is in good condition.  Or it is going to be caused by an issue your Sierra’s oil level, viscosity, pressure, or cleanliness.  

This code is important because it ensures proper oil flow.


These are some of the leading causes for P0521.

  • No oil or low-quality The first thing that you should do is check to see if there is oil in your Sierra’s engine at all.  The crankcase can give P0521 when it is depleted or very low in oil.  This can be fixed by filling the oil with more, clearing the code and resetting the crankcase.  
  • Low oil pressure This code may appear if the oil pressure is too low.  This code should be visible with an oil light or low oil pressure readings from the gauge.  GMC Sierra Low oil pressure diagnosis
  • Wrong or dirty oil- If there hasn’t been an oil change for a long enough period of time, it can interfere with the oil pressure sensor, even if there still is oil pressure. 
  • Oil Pressure Sensor Wiring - P0521 means that the signal coming from the oil pressure sensors is not within the range.  This is a signal that there are wiring problems.
  • Oil Pressure Sensor Although the P0521 problem is most likely caused by the oil pressure sensor, it is worth looking into the wiring and oil first.


GMC Sierra Diagnosis P0521

This P0521 diagnosis order is written (like everything on the site) for the DIY guy who doesn’t have a lot of specialty tools. 

1. Take a look at the Oil

Check the oil level first.  If it’s full make sure that it looks healthy.  You may have an oil pressure sensor problem if the lid appears blackened or grimy.


2. Examine the Wiring Harness

Next, you should inspect the wiring harness running from the oil pressure sensor to the PCM.  It could also be the cause of P0521.  Don’t forget to make sure that the harness is still plugged into the sending unit and undamaged.  

One of the most common reasons this code is thrown is because wiring problems are common.  The wiring is close to the Sierra’s exhaust and can get brittle over time.


3. Manually check your oil pressure

If the wiring and oil levels are in good condition, you can manually test the oil pressure.  A Oil pressure can be manually adjusted gauge to see if it’s where it needs to be.  These gauges are very easy to use.  Here’s a Great YouTube Video This will demonstrate how you can use the gauge to test.

If the oil pressure from the gauge reads ok, you’ll need to replace the oil pressure sending unit/sensor.  They’re cheap.  If there is low or no oil pressure, you’ll need to figure out why.



It’s relatively straightforward diagnosing P0521.  This is made easier by knowing the exact oil pressure.  If you have any suggestions, please leave them below. Good luck with your GMC Sierra.

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