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MID 128 PID 26 FMI 3Severity: Warning

MID 128 PID 26 FMI 3: Fan Speed Sensor - Voltage Above Normal (Volvo)

Quick Answer

When MID 128 PID 26 FMI 3 sets, the ECM is indicating a fault condition involving the component or system named in the code.

OEM definitions and thresholds can vary, so confirm the exact meaning with service information or a capable diagnostic tool.

Code
MID 128 PID 26 FMI 3
Severity
Warning
Applies to
Check OEM application
Can I drive?
A short trip may be possible if the truck is operating normally and there is no stop-engine or critical warning. Diagnose MID 128 PID 26 FMI 3 soon and use the live-data and circuit checks below to determine how urgent the fault is.

Common Symptoms

  • Higher fuel consumption
  • Fan operates as on/off only (100% when cooling needed)

Likely Causes

  • Cooling Fan Speed (CFS) sensor failure
  • Faulty sensor harness
  • Short circuit to high source
  • Short circuit to low source

Quick Diagnostic Checks

Check first
  1. 1Scan for companion codes and record freeze-frame data (if available).
  2. 2Inspect connectors, harness rub points, and pins for corrosion.
  3. 3Check for other active faults that may be upstream causes (communications, sensors, air/fuel issues).
  4. 4After repair, clear codes and confirm the fault stays inactive after a drive cycle.

Common Fixes

  • Inspect Cooling Fan Speed sensor and wiring
  • Check for shorts or opens in harness
  • Replace faulty CFS sensor if needed
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FitmentsMeaningHow it worksImagesDetailed guideOEM/Engine notesLocation hintScan tool checksStep-by-step diagnosticDecision guideMisdiagnosisDerate/inducementChecklistToolsPreventionCommunity discussionsCommunity videosCommunity notesShare

What does MID 128 PID 26 FMI 3 mean?

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When MID 128 PID 26 FMI 3 sets, the ECM is indicating a fault condition involving the component or system named in the code.

OEM definitions and thresholds can vary, so confirm the exact meaning with service information or a capable diagnostic tool.

Treat repeat faults as ACTIVE and diagnose using live data, wiring checks, and basic inspections before replacing parts.

How this system works (why MID 128 PID 26 FMI 3 happens)

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

A stored code can come from a temporary event, while a repeatable active code gives you a much better diagnostic target.

The goal is to identify whether the failure is electrical, mechanical, sensor-related, or caused by another upstream fault before replacing components.

Detailed guide for MID 128 PID 26 FMI 3

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MID 128 PID 26 FMI 3 - Fan Speed Percent

**Fault Type:** Voltage above normal or shorted to high source

Fault Condition

  • Missing signal from Fan Speed Sensor
  • Short Circuit + (Measuring line)
  • Short Circuit - (Measuring line)
  • Open Circuit (Measuring line)
  • Open Circuit (Ground line)

Possible Symptoms

  • Higher fuel consumption
  • Fan will work as on/off only (100% fan speed if cooling is needed)

Possible Causes

  • Cooling Fan Speed (CFS) sensor failure
  • Faulty Cooling Fan Speed (CFS) sensor harness

Sensor Location

The Cooling Fan Speed (CFS) Sensor is located in the fan drive on the front of the engine.

Diagnostic Tips

1. Inspect the Cooling Fan Speed sensor and its wiring harness

2. Check for shorts to battery or ground in the measuring line

3. Verify continuity in both measuring and ground lines

4. Use Premium Tech Tool (PTT) to monitor fan speed signal

5. On engines with electronically controlled viscous fan drive, the electronic fan drive contains a Hall effect speed sensor

Location hint for MID 128 PID 26 FMI 3

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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 26 FMI 3

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  • Check status: ACTIVE vs INACTIVE / stored and review freeze-frame data when available.
  • Record companion codes and diagnose the earliest or most upstream fault first.
  • Compare live sensor values with expected operating conditions and inspect wiring and connectors.
  • Clear and re-test under similar conditions only after completing the initial inspection.

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 26 FMI 3

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  1. Visual inspection (fast wins). Harness routing, connector pins, hoses/lines (if applicable), and obvious damage.
  2. Check fluid sanity. Not applicable unless your fault is fluid-related. Focus on inspections + repeatability.
  3. Confirm the fault is ACTIVE. Stored-only faults can be old events.
  4. Review live data and circuit values. Use OEM diagnostic software (examples: Cummins INSITE, Detroit DDDL, PACCAR DAVIE4) or a capable heavy-duty scan tool to review the affected signal, commanded/actual values when applicable, and related sensor data.
  5. Verify after repair. Clear codes, road test, confirm it stays inactive.

Decision guide (fast path) for MID 128 PID 26 FMI 3

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  • If the fault is STORED only → inspect, clear, and monitor. If it returns, treat it as active.
  • If ACTIVE → inspect the named component, connector, harness, supply/ground, and live-data plausibility.
  • If multiple codes are present → diagnose the earliest or most upstream code first.

Common misdiagnosis mistakes for MID 128 PID 26 FMI 3

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  • Replacing sensors or components without checking wiring, grounds, connector fitment, and companion codes.
  • Clearing the fault repeatedly without confirming whether it is ACTIVE and repeatable.
  • Missing an upstream fault that is causing the displayed code as a secondary symptom.

Will MID 128 PID 26 FMI 3 cause derate or inducement?

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A warning fault may allow continued operation, but the effect depends on the failed component and any companion faults.

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

“Do this first” checklist for MID 128 PID 26 FMI 3

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  • Scan for companion 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 26 FMI 3

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  • 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 live data and supported diagnostic tests

Prevention tips to avoid repeat MID 128 PID 26 FMI 3

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  • 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 26 FMI 3

  1. Inspect connectors and harness near the affected component for corrosion, rubbing, or loose pins.
  2. Check for upstream causes (communications, sensors, air/fuel issues).
  3. 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 and review companion codes and live data before replacing parts.

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Mechanic community notes for MID 128 PID 26 FMI 3

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