Massey Ferguson 520391.31 – Engine Control Unit (ECU): Air mass sensor start test value not plausible (too high)

During the engine's initial key-on/start-up self-test, the Engine Control Unit (ECU) checks the baseline voltage/reading of the Air Mass Flow (MAF) sensor. This code triggers when the sensor reports a value that is significantly higher than the expected atmospheric/zero-flow baseline, indicating an implausible reading before the engine is even drawing a substantial volume of air.

• 4710
• 7724S
• 5712SL
• 7722S
• 7722
• 5713SL
• 7720S
• 7716
• 7718
• T4
• 7715
• 5710SL
• 7726
• 4708
• 7726S
• 4707
• 4709
• 5711SL
• 7718S
• 7719
• 7714
• 7716S
• T4 (2015-2021)
• 7720
• Tier 4 (2015-2021)
• 7715S
• 7724
• Massey Ferguson 5S Series
• Massey Ferguson 6S Series
• Massey Ferguson 7700 Series
• MF 7715
• MF 7718
• Massey Ferguson 8400 Series
• Other AGCO equipment utilizing Tier 3, Tier 4, and Stage V AGCO Power engines

1. 1

   Verify battery voltage, as a voltage spike or low voltage condition during the ECU's start test can cause the sensor's analog voltage output to skew too high.

2. 2

   Inspect the wiring harness leading to the Air Mass Flow Sensor for chafed wires causing a short-to-power, and check connector pins for corrosion, moisture, or poor tension.

3. 3

   Check the air filter and the intake tract for severe blockages that might skew start-test calibrations.

4. 4

   Remove the MAF sensor and inspect the sensing element for dust, oil blow-by, or debris on the hot wire/film.

1. 1

   Power Down

   Turn off the tractor ignition and remove the key. Disconnect the negative battery terminal to prevent electrical spikes during component replacement.

2. 2

   Locate the Sensor

   Locate the Air Mass Flow Sensor, which is mounted on the main air intake pipe between the air filter housing and the turbocharger inlet.

3. 3

   Disconnect Harness

   Depress the locking tab on the sensor's electrical connector and carefully pull it off.

4. 4

   Remove the Sensor

   Remove the retaining screws (usually Torx or Allen head) holding the sensor to the intake housing. Carefully pull the sensor straight out of the housing.

5. 5

   Install the New Sensor

   Insert the new Air Mass Flow Sensor into the housing. Ensure the O-ring is seated properly to prevent unmetered air leaks, and verify that the directional arrow on the sensor matches the direction of airflow (towards the engine).

6. 6

   Reassembly

   Tighten the retaining screws to snug (do not overtighten, as the housing is often plastic). Reconnect the electrical harness until it clicks.

7. 7

   Reconnect Battery & Clear Codes

   Reconnect the battery. Use an AGCO Electronic Diagnostic Tool (EDT) to clear the inactive 520391.31 fault code.

8. 8

   Verification

   Cycle the key to the ON position to allow the ECU to perform a new start test. Start the engine and verify that the code does not return and that the engine is no longer in a power-derate state.