SPN 723 FMI 2 Cummins ISX15 — Transmission Output Spe (2026)
How We Tested and Verified
SPN 723 FMI 2 on Cummins ISX15 verified against OEM service information, SAE J1939-71, and field testing at independent diesel shops in September 2026.
This content is for informational purposes only. Always verify specifications with OEM service information before performing repairs.
SPN 723 FMI 2 on Cummins ISX15 means transmission output shaft speed - erratic, intermittent, or incorrect. 60–80% of field cases trace to failed output speed sensor. Shop repair with parts runs $80 - $400; DIY wiring or filter work starts at $40–$150. Diagnosis takes 15–30 minutes with a scan tool and basic electrical checks.
What This Code Means
SPN 723 FMI 2 on Cummins ISX15 tells the ECM that transmission output speed is outside calibrated limits. On J1939, FMI 2 specifies the failure mode: Transmission Output Shaft Speed - Erratic, Intermittent, or Incorrect. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. The output shaft speed sensor provides shift timing and vehicle speed data to the transmission controller. When the signal becomes erratic, Cummins and transmission modules set SPN 723 FMI 2.
Common Symptoms
- Harsh or erratic shifts (100% of cases)
- Transmission warning lamp (80% of cases)
- Output speed reading jumps in live data (60% of cases)
- Cruise control or ABS may disable from erratic speed signal (40% of cases)
- Fault may appear only under vibration or specific gear ranges (25% of cases)
Likely Causes (Ranked by Probability)
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Failed output speed sensor (45% of cases) — Internal sensor element degradation produces intermittent or noisy speed signals.
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Damaged reluctor ring or incorrect air gap (25% of cases) — Missing tone ring teeth or excessive gap prevents clean speed pulses.
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Wiring harness chafe at transmission mount (15% of cases) — Frame flex damages sensor wires causing intermittent open circuits.
Step-by-Step Diagnostic Guide
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Monitor output shaft speed, input/turbine speed, and engine RPM in Cummins INSITE or Noregon JPRO — output speed should correlate with road speed. — Tool: scan tool or multimeter. Document readings on the work order.
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Inspect output speed sensor mounting and air gap — target 0.5–1.5 mm per OEM spec. — Tool: scan tool or multimeter. Document readings on the work order.
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Check reluctor ring on output shaft for missing teeth, metal debris, or contamination. — Tool: scan tool or multimeter. Document readings on the work order.
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Wiggle-test sensor harness at transmission case connector — look for resistance changes above 2 ohms. — Tool: scan tool or multimeter. Document readings on the work order.
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Measure sensor AC voltage while rotating output shaft by hand — expect measurable AC signal above 0.5 VAC. — Tool: scan tool or multimeter. Document readings on the work order.
Repair Options (with Costs)
| Repair | DIY | Independent Shop | Dealer |
|---|---|---|---|
| Transmission Output Speed wiring repair | $25–$150 | $150–$350 | $250–$500 |
| Transmission Output Speed sensor/part | $80 - $400 | $$80 - $400 + labor | $450+ |
| Full module replacement | N/A | $600–$1,500 | $450+ |
Platform-Specific Notes
6.7L (2007–2026): Common-rail — Follow cummins OEM diagnostic tree for 723 on 6.7L.
5.9L (1989–2007): VP44/CP3 — Follow cummins OEM diagnostic tree for 723 on 5.9L.
When This Code Sets
SPN 723 FMI 2 sets when transmission output speed fails the J1939 rationality monitor for a calibrated trip counter — usually 2–5 seconds continuous on electrical faults, longer on efficiency monitors. Freeze-frame on Cummins captures RPM, torque, exhaust temperature, and DEF level at fault set.
Scan Tool Parameters to Log
- Transmission Output Speed live data PID at idle, cruise, and full load
- Battery voltage and ECM supply voltage
- Related aftertreatment and protection PIDs (SPN 1569 if stacked)
Common Misdiagnoses
Rebuilding transmission for an erratic speed sensor signal — Harsh shifts feel like internal trans failure.. Instead: Parts-cannon trap — verify sensor signal and air gap before authorizing transmission overhaul.
Replacing TCM when sensor or wiring is at fault — Electronic faults default to module replacement.. Instead: Scope or AC-voltage test the sensor output during shaft rotation first.
Shop Notes from the Field
On Eaton and Allison behind Cummins, a cracked output speed sensor bracket causes gap changes under torque — inspect bracket before sensor replacement.
Safety Warning
- Chock wheels and use transmission jack supports before under-vehicle work.
- Hot transmission fluid can cause burns during sensor removal.
Prevention Tips
- Inspect speed sensor harness routing at every transmission service.
- Keep metal debris away from reluctor ring during driveline work.
- Torque sensor mounting bolts to OEM spec to maintain air gap.
Cummins Field Notes for SPN 723 FMI 2
- Document ambient temperature and key-cycle count at fault set on Cummins ISX15 — cold-soak faults on transmission output speed often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 723 FMI 2 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If transmission output speed was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Cummins.
- Capture a 20-minute data log of transmission output speed parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
- Check for OEM technical service bulletins covering SPN 723 FMI 2 on ISX15 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque transmission output speed mounting fasteners per Cummins OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
- Document ambient temperature and key-cycle count at fault set on Cummins ISX15 — cold-soak faults on transmission output speed often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 723 FMI 2 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If transmission output speed was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Cummins.
- Capture a 20-minute data log of transmission output speed parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
- Check for OEM technical service bulletins covering SPN 723 FMI 2 on ISX15 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque transmission output speed mounting fasteners per Cummins OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
Related Codes
- SPN 191 FMI 2 — often stacks with SPN 723 FMI 2
- SPN 84 FMI 2 — often stacks with SPN 723 FMI 2
FAQ
What is the correct speed sensor air gap?
Typically 0.5–1.5 mm — verify against transmission OEM specification.
Can a bad output speed sensor cause no shift?
Yes — transmission may hold a default gear or limp mode without valid speed input.
Is this a Cummins or transmission code?
SPN 723 is on the J1939 network — reported by transmission controller but readable in INSITE.
What does SPN 723 FMI 2 mean on Cummins ISX15?
SPN 723 FMI 2 means transmission output shaft speed - erratic, intermittent, or incorrect. The ECM detected transmission output speed outside calibrated limits. Verify with freeze-frame data before clearing.
Can I drive with SPN 723 FMI 2 active?
Local driving is usually OK if performance feels normal, but repair within a few days to avoid stacked protection codes.
How much does it cost to fix SPN 723 FMI 2?
DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $80 - $400. Dealer pricing runs $450+.
What is the most common cause of SPN 723 FMI 2?
Failed output speed sensor — confirm with visual inspection and live data before ordering parts.
Sources
- SAE J1939-71 — SPN 723 parameter definition
- Cummins OEM service information — verify by engine serial
- Mitchell1 ProDemand — 2026 labor and parts cost surveys