Can I drive with MID 128 PID 84 FMI 9?

Usually yes for a short distance, but fix it soon. Warning faults like MID 128 PID 84 FMI 9 commonly escalate into torque derate if ignored. The scan-tool checks below help determine urgency.

How do I fix MID 128 PID 84 FMI 9?

Start with: Check VECU power and ground. Then follow the diagnostic steps and scan-tool checks on this page.

What tools help diagnose MID 128 PID 84 FMI 9?

A quality J1939 scan tool and, when available, OEM diagnostic software (examples: Cummins INSITE, Detroit DDDL, PACCAR DAVIE4) for actuator tests and live data.

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Fitments Meaning How it works Images Detailed guide OEM/Engine notes Location hint Scan tool checks Step-by-step diagnostic Decision guide Misdiagnosis Derate/inducement Checklist Tools Prevention Community notes Share

Level: Warning

MID 128 PID 84 FMI 9: Vehicle Speed - Abnormal Update Rate (Volvo)

MID 128 PID 84 FMI 9: Vehicle Speed - Abnormal Update Rate (Volvo). Symptoms (1), causes (2), and fixes (4). What to check first to prevent derate and downtime.

Code	MID 128 PID 84 FMI 9
Severity	Warning
Applies to	Varies by OEM configuration (confirm your exact calibration)
Can I drive?	Usually yes for a short distance, but fix it soon. Warning faults like MID 128 PID 84 FMI 9 commonly escalate into torque derate if ignored. The scan-tool checks below help determine urgency.

Symptoms

Likely causes

Fixes

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What does MID 128 PID 84 FMI 9 mean?

When MID 128 PID 84 FMI 9 sets, the ECM is indicating a fault condition that affects performance, protection strategy, or emissions (depending on calibration).

OEM definitions can vary slightly, so confirm your exact meaning with your service manual or diagnostic tool.

Treat repeat faults as ACTIVE and diagnose using a scan tool and basic inspections before replacing parts.

How this system works (why MID 128 PID 84 FMI 9 happens)

Most modern fault logic is based on expected vs actual sensor readings, timing windows, and plausibility checks.

A single event might store a code, but repeated events across drive cycles are what typically trigger warnings, derate strategies, or inducement (emissions systems).

Your goal is to identify the failure mode (electrical, mechanical, sensor drift, or upstream cause) rather than “parts cannon” replacement.

Detailed guide for MID 128 PID 84 FMI 9

MID 128 PID 84 FMI 9 - Vehicle Speed

**Fault Type:** Abnormal update rate

Fault Condition

Missing signal from VECU (Vehicle Electronic Control Unit)

Possible Symptoms

Engine derate

Possible Causes

J1708 vehicle speed message does not exist (VECU error)
Communication failure between EMS and VECU
VECU not broadcasting vehicle speed data

System Information

The Vehicle Electronic Control Unit (VECU) directly monitors the Vehicle Speed Sensor (VSS). The VECU broadcasts this data on the serial data lines to other ECUs including the EMS.

Diagnostic Tips

1. Check VECU for fault codes

2. Verify VECU power and ground connections

3. Inspect J1939 and J1708 data link wiring for damage

4. Check termination resistors on data links

5. Verify vehicle speed sensor operation and wiring to VECU

6. Use Premium Tech Tool (PTT) to monitor network communication

Location hint for MID 128 PID 84 FMI 9

Component location varies by OEM and chassis. Use your engine’s service manual to confirm the sensor/valve location before replacing parts.

Tip: Inspect connectors and harness routing before replacing parts.

Scan tool checks for MID 128 PID 84 FMI 9

Check status: ACTIVE vs INACTIVE / stored. If it’s not active, look at last occurrence conditions (freeze-frame if available).
Record companion codes. Fixing the “first” code often makes others disappear.
Clear and re-test under similar conditions. If it returns quickly, you have a repeatable fault (good for diagnosis).

If your tool can’t run actuator tests, you can still diagnose a lot with careful inspection + repeatability testing.

Step-by-step diagnostic for MID 128 PID 84 FMI 9

Visual inspection (fast wins). Harness routing, connector pins, hoses/lines (if applicable), and obvious damage.
Check fluid sanity. Not applicable unless your fault is fluid-related. Focus on inspections + repeatability.
Confirm the fault is ACTIVE. Stored-only faults can be old events.
Run actuator tests (if supported). Use OEM diagnostic software (examples: Cummins INSITE, Detroit DDDL, PACCAR DAVIE4) (or a capable J1939 tool) to command tests when available.
Verify after repair. Clear codes, road test, confirm it stays inactive.

Decision guide (fast path) for MID 128 PID 84 FMI 9

If the fault is STORED only → inspect, clear, and monitor. If it returns, treat it as active.
If ACTIVE → check connector/harness first → then confirm sensor values and plausibility → then consider component testing.
If you have multiple codes → diagnose the earliest/most upstream code first.

Common misdiagnosis mistakes for MID 128 PID 84 FMI 9

Replacing sensors without checking wiring, grounds, and connector pin fitment.
Clearing codes repeatedly without confirming whether the fault is ACTIVE and repeatable.
Missing companion codes that point to the real upstream problem.

Will MID 128 PID 84 FMI 9 cause derate or inducement?

Warning aftertreatment faults often start as an emissions warning, then progress to torque derate if the fault remains active over multiple drive cycles. If ignored, many platforms can eventually enter inducement and severe speed limiting.

Exact behavior depends on ECM calibration and which companion faults are active.

“Do this first” checklist for MID 128 PID 84 FMI 9

Scan for companion/derate codes and record freeze-frame data (if available).
Inspect connectors, harness rub points, and pins for corrosion.
Check for other active faults that may be upstream causes (communications, sensors, air/fuel issues).
After repair, clear codes and confirm the fault stays inactive after a drive cycle.

Tools needed to diagnose MID 128 PID 84 FMI 9

Basic hand tools + good lighting for inspection
Multimeter for connector/power/ground checks (if wiring suspected)
OEM diagnostic software (examples: Cummins INSITE, Detroit DDDL, PACCAR DAVIE4) (or equivalent scan tool) for actuator tests and live data

Prevention tips to avoid repeat MID 128 PID 84 FMI 9

Fix small wiring/connector issues early before they become repeat faults.
During service, inspect harness routing and clamp points near heat/vibration.
Don’t ignore intermittent faults. “Erratic” usually gets worse.

What to check first for MID 128 PID 84 FMI 9

Inspect connectors and harness near the affected component for corrosion, rubbing, or loose pins.
Check for upstream causes (communications, sensors, air/fuel issues).
Check for other active faults that may be upstream causes (communications, sensors, air/fuel issues).

If the fault repeats after clearing, diagnose it as ACTIVE. Many faults escalate into derate when ignored.

Related fault codes

No related faults found yet. Commonly seen with: SPN 5246, SPN 1569, SPN 4364, SPN 3719.

Mechanic community notes for MID 128 PID 84 FMI 9

Real-world tips from technicians. Submissions are moderated to keep spam and “my cousin fixed it with duct tape” content out.

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