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SPN 84 FMI 6 – VSS Anti-Tamper Detection via ABS Vehicle Speed Comparison Fault Code (Symptoms, Causes & Fix)

The SPN 84 FMI 6 fault code is a diesel engine diagnostic trouble code indicating a discrepancy between the Vehicle Speed Sensor (VSS) and the Anti-lock Braking System (ABS) wheel speed readings. This code is triggered when the Engine Control Module (ECM) detects a significant difference in speed data, suggesting potential tampering or a fault in the speed sensing components.

What SPN 84 FMI 6 Means

The SPN 84 FMI 6 fault code represents an anti-tamper detection issue where the ECM compares the vehicle speed data from the VSS and the ABS. If the readings don't align within acceptable parameters, the ECM flags this as a fault, indicating possible tampering or a malfunction in the speed sensing systems.

Common Causes of This Fault Code

  • Faulty Vehicle Speed Sensor (VSS)
  • Damaged or corroded VSS wiring harness
  • Faulty ABS wheel speed sensor
  • Incorrect tire size or tire pressure causing speed discrepancies
  • Faulty ABS module

Symptoms Drivers May Notice

When the SPN 84 FMI 6 fault code is active, drivers may notice the ABS warning light illuminated on the dashboard. There might also be erratic speedometer readings or a loss of cruise control functionality. In some cases, the vehicle may enter a limp mode to prevent further damage.

How Technicians Diagnose This Fault

Technicians begin diagnosing this fault by performing a visual inspection of the VSS and ABS components. They check for any obvious signs of damage or corrosion in the wiring harness and connectors. Next, they verify the power supply and ground circuits to the VSS and ABS sensors using a multimeter. If necessary, they test the 5-volt reference circuits.

Using diagnostic software, technicians will check for additional ABS fault codes and perform ABS module tests. They will also verify the tire sizes and pressures to ensure they match the vehicle's specifications. Signal readings from the VSS and ABS sensors are checked using a scan tool or oscilloscope to confirm proper operation.

Common Misdiagnosis

A common misdiagnosis involves replacing the VSS without thoroughly inspecting the wiring harness for issues. Technicians may also overlook ABS wheel speed sensor faults or ignore discrepancies in tire size or pressure, leading to unnecessary part replacements.

Can You Keep Driving?

While the vehicle may still be drivable with the SPN 84 FMI 6 fault code active, it is not advisable to ignore it. The fault can lead to inaccurate speed readings, affecting systems like ABS and cruise control. In some cases, the vehicle may enter a limp mode, reducing performance to protect the engine.

Mechanic's Diagnostic Tip

Always verify the tire sizes and pressures first when dealing with SPN 84 FMI 6. Mismatched tires can cause speed discrepancies that trigger this fault code. This simple check can save time and prevent unnecessary sensor replacements.

Frequently Asked Questions

Q: Can a faulty ABS sensor trigger SPN 84 FMI 6?
A: Yes, a faulty ABS sensor can cause discrepancies in speed readings, leading to this fault code.

Q: Will replacing the VSS fix the SPN 84 FMI 6 fault code?
A: Not necessarily. It's crucial to perform a full diagnostic to ensure the VSS is the root cause before replacement.

Q: Can incorrect tire pressure cause SPN 84 FMI 6?
A: Yes, incorrect tire pressure can lead to speed discrepancies, triggering the fault code.

Other FMI Codes for SPN 84