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SPN 84 FMI 20 – Vehicle Speed Sensor Drifted High Error (VSS signal not plausible) Fault Code (Symptoms, Causes & Fix)

The SPN 84 FMI 20 fault code is a diesel engine diagnostic trouble code indicating a Vehicle Speed Sensor (VSS) drifted high error, meaning the VSS signal is not plausible. This code is triggered when the Engine Control Module (ECM) detects an unexpected or inconsistent signal from the VSS, which is crucial for accurate speed readings.

What SPN 84 FMI 20 Means

SPN 84 FMI 20 specifically relates to the Vehicle Speed Sensor's inability to provide a reliable signal to the ECM. The ECM relies on the VSS to monitor vehicle speed accurately. When the signal is deemed implausible, it can affect various systems that depend on speed data, such as transmission shifting and cruise control.

Common Causes of This Fault Code

  • Failed Vehicle Speed Sensor
  • Wiring harness damage or corrosion
  • Poor electrical connections at the VSS or ECM
  • ECM signal processing issues
  • Voltage reference faults

Symptoms Drivers May Notice

When the SPN 84 FMI 20 fault code is active, drivers may notice erratic speedometer readings or the speedometer not functioning at all. Additionally, there might be issues with transmission shifting, cruise control malfunction, or even unexpected engine derate.

How Technicians Diagnose This Fault

Technicians begin diagnosing the SPN 84 FMI 20 fault code with a visual inspection of the VSS and its mounting for any physical damage or debris. Next, they perform a continuity and resistance check on the VSS wiring harness to identify any breaks or shorts. Proper voltage supply and ground at the VSS connector are verified using a multimeter. An oscilloscope may be used to check the VSS signal waveform for irregularities. Finally, the ECM is inspected for any software updates or calibration issues that might affect signal processing.

Common Misdiagnosis

A common misdiagnosis for SPN 84 FMI 20 is replacing the VSS without thoroughly checking the wiring integrity. Technicians might also mistakenly assume a mechanical issue with the transmission or differential, or overlook potential ECM software updates or calibration errors.

Can You Keep Driving?

While it may be possible to continue driving with the SPN 84 FMI 20 fault code, it is not advisable. The inaccurate speed readings can lead to improper transmission shifting and other drivability issues. In some cases, the ECM may initiate an engine derate or protection mode, which could limit vehicle performance.

Mechanic's Diagnostic Tip

When diagnosing SPN 84 FMI 20, always verify the integrity of the wiring harness and connectors before replacing the VSS. Many issues stem from poor connections or damaged wiring rather than the sensor itself.

Frequently Asked Questions

Q: What tools are essential for diagnosing SPN 84 FMI 20?
A: A multimeter and an oscilloscope are crucial for checking voltage, continuity, and signal waveform.

Q: Can a software update fix SPN 84 FMI 20?
A: Yes, if the issue is related to ECM signal processing, a software update or reflash might resolve the fault.

Q: Is it necessary to replace the VSS if SPN 84 FMI 20 is active?
A: Not necessarily. Always confirm the sensor's failure through electrical testing before replacement.

Other FMI Codes for SPN 84