P2040 OBD-II Trouble Code: Reductant Injection Air Pressure Sensor Circuit High Input

Check Engine Light

P2040 code definition

Reductant Injection Air Pressure Sensor Circuit High Input*

*Definition could differ depending on manufacturer code usage, for example, “Oil Quality Sensor ME-SFI” (Mercedes)

What the P2040 code means

P2040 is an OBD-II generic code for the Reductant Injection Air Pressure Sensor Circuit High Input. However, the code is used by manufacturers at their discretion, and they can change the usage of the code to whatever they want. Since this code definition is not used by most manufactures, some, such as Mercedes, are using it for another failure.

Mercedes uses P2040 as an oil quality sensor ME-SFI code for the oil quality signal being implausible, not credible, or out of specified duty cycle range of smaller than 17% to larger than 83%, while checking the oil dielectric content for excess water in the oil.

What causes the P2040 code?

  • The Engine Control Module (ECM) monitors the engine oil quality sensor duty cycle and detects a reading to be below 17% or above 83% (Mercedes)

  • The dielectric reading goes above 5.8 dielectric sensor output to the ECM (indicates the oil contains excessive water*)

*Normal readings from the sensor for oil without water would be between 2.3 and 4.0.

What are the symptoms of the P2040 code?

  • The Check Engine Light will not be illuminated, and the code is set in ECM memory as a oil quality fault.

How does a mechanic diagnose the P2040 code?

  • Scans codes and documents the code in the ECM

  • Scans for codes other than P2040 in the ECM and diagnoses those codes as per the manufacturer's pinpoint test

  • Clears the ECM code P2040 after engine oil and filter change

  • Rescans the ECM oil sensor for a reading to below 4.0 dielectric

Common mistakes when diagnosing the P2040 code

  • Clearing ECM memory codes before checking the oil sensor data in memory
  • Failing to clear ECM codes after the P2040 codes are fixed

How serious is the P2040 code?

  • Code P2040 is an indication that the engine oil has too much water moisture content in the oil.

  • The excess water in the oil could lead to lack of lubrication to the bearings and engine component failure if the engine is run for a long time with excessive water in the oil without an oil change.

What repairs can fix the P2040 code?

Additional comments for consideration regarding the P2040 code

Code P2040 is used mostly by manufacturers with diesel engine oil quality sensors and not for the OBD-II generic code description Reductant Injection Air Pressure Sensor Circuit High Input.

Need help with a P2040 code?

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