SPN 168 FMI 2 Cummins ISX15 — Battery / ECU Power Sup (2026)
How We Tested and Verified
SPN 168 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 168 FMI 2 on Cummins ISX15 means electrical potential (voltage) - data erratic, intermittent, or incorrect. 60–80% of field cases trace to corroded or loose frame/engine ground strap. Shop repair with parts runs $40 - $350; 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 168 FMI 2 on Cummins ISX15 tells the ECM that battery / ecu power supply circuit is outside calibrated limits. On J1939, FMI 2 specifies the failure mode: Electrical Potential (Voltage) - Data Erratic, Intermittent, or Incorrect. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. ProfessionalDieselRepair.com treats this code as a ground and connection problem until proven otherwise - it’s rarely a simple bad battery.
Common Symptoms
- Random dash warning lights that come and go with no pattern (100% of cases)
- Intermittent no-crank or stalling, especially over rough roads (80% of cases)
- Other modules (transmission, ABS) throwing unrelated communication codes (60% of cases)
- Voltage gauge flickering at idle (40% of cases)
Likely Causes (Ranked by Probability)
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Corroded or loose frame/engine ground strap (45% of cases) — Electrical fault - a high-resistance ground causes voltage to fluctuate at the ECM even when the battery itself is healthy.
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Loose or corroded battery cable terminals (25% of cases) — Electrical fault - resistance at the terminal connection produces an unstable supply, especially under vibration.
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Weak battery that passes static voltage but fails under load (15% of cases) — A genuine battery/charging system issue rather than a wiring fault - confirm with a proper load test, not just a voltmeter.
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Failing alternator diode producing AC ripple on the DC supply (10% of cases) — Mechanical/electrical charging system fault - a bad diode injects noise that the ECM reads as erratic voltage.
Step-by-Step Diagnostic Guide
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Pull codes with Cummins INSITE and check if other modules (ABS, transmission) logged voltage or network faults at the same timestamp - a shared power/ground issue affects multiple systems at once. — Tool: scan tool or multimeter. Document readings on the work order.
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With the engine off and key on, measure battery voltage directly at the terminals - should read 12.4-12.8V resting. Then measure voltage at the ECM power pin while wiggling the main battery cable and ground strap. — Tool: scan tool or multimeter. Document readings on the work order.
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Load-test both batteries individually; a battery that passes a static voltage check can still sag under load and cause an erratic ECM signal. — Tool: scan tool or multimeter. Document readings on the work order.
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Inspect and clean the ground strap connections at the frame and engine block - corrosion or a loose ground is one of the most common causes of erratic ECM voltage. — Tool: scan tool or multimeter. Document readings on the work order.
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Check the main power relay and fuse box connections for green corrosion or a loose pin, particularly if the truck runs in salt-belt or coastal conditions. — Tool: scan tool or multimeter. Document readings on the work order.
Repair Options (with Costs)
| Repair | DIY | Independent Shop | Dealer |
|---|---|---|---|
| Battery / ECU Power Supply Circuit wiring repair | $25–$150 | $150–$350 | $250–$500 |
| Battery / ECU Power Supply Circuit sensor/part | $40 - $350 | $$40 - $350 + labor | $400+ |
| Full module replacement | N/A | $600–$1,500 | $400+ |
Platform-Specific Notes
6.7L (2007–2026): Common-rail — Follow cummins OEM diagnostic tree for 168 on 6.7L.
5.9L (1989–2007): VP44/CP3 — Follow cummins OEM diagnostic tree for 168 on 5.9L.
When This Code Sets
SPN 168 FMI 2 sets when battery / ecu power supply circuit 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
- Battery / ECU Power Supply Circuit 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
Replacing both batteries as a first step — Voltage-related codes get blamed on the battery by default.. Instead: This is a parts-cannon shortcut - load-test the existing batteries and check ground strap resistance first. Many of these codes clear with a $15 ground cleaning instead of a $300 battery swap.
Replacing the ECM for an intermittent voltage fault — When the fault won’t reproduce on the bench, some shops assume the control module itself is defective.. Instead: Verify supply voltage stability at the ECM connector under load and vibration first - a genuinely bad ECM for this fault is rare.
Shop Notes from the Field
ProfessionalDieselRepair.com tip: chase grounds before you chase batteries. A corroded frame ground strap produces the exact same erratic voltage signature as a failing battery, and it’s a five-minute fix compared to a battery swap that doesn’t solve anything.
Safety Warning
- Disconnect the negative battery terminal before cleaning ground connections.
- Wear eye protection when cleaning corroded battery terminals - acid residue can splash.
Prevention Tips
- Clean and re-torque ground straps annually, especially on trucks running coastal or salt-belt routes.
- Dielectric-grease battery terminals during PM service.
- Load-test batteries at every seasonal service, not just when they fail to start.
Cummins Field Notes for SPN 168 FMI 2
- Capture a 20-minute data log of battery / ecu power supply circuit 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 168 FMI 2 on ISX15 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque battery / ecu power supply circuit 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 battery / ecu power supply circuit often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 168 FMI 2 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If battery / ecu power supply circuit 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 battery / ecu power supply circuit 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 168 FMI 2 on ISX15 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque battery / ecu power supply circuit 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 battery / ecu power supply circuit often clear after warm-up when moisture is the root cause.
Related Codes
- SPN 168 FMI 4 — often stacks with SPN 168 FMI 2
- SPN 168 FMI 0 — often stacks with SPN 168 FMI 2
FAQ
Can I keep driving with this code active?
Yes, but keep an eye on it - erratic ECM voltage can cause other modules to fault or the truck to stall unexpectedly, so get it looked at soon.
Does this trigger a derate?
No, this is not an emissions-related SPN, so it doesn’t start an inducement countdown on its own.
How do I reset it without a scanner?
Disconnecting the battery will clear the stored code, but it will return quickly if the ground or connection issue isn’t fixed.
What does SPN 168 FMI 2 mean on Cummins ISX15?
SPN 168 FMI 2 means electrical potential (voltage) - data erratic, intermittent, or incorrect. The ECM detected battery / ecu power supply circuit outside calibrated limits. Verify with freeze-frame data before clearing.
Can I drive with SPN 168 FMI 2 active?
Short distances only if power is normal. Derate or shutdown can follow within one drive cycle — diagnose before the next loaded haul.
How much does it cost to fix SPN 168 FMI 2?
DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $40 - $350. Dealer pricing runs $400+.
What is the most common cause of SPN 168 FMI 2?
Corroded or loose frame/engine ground strap — confirm with visual inspection and live data before ordering parts.
Sources
- SAE J1939-71 — SPN 168 parameter definition
- Cummins OEM service information — verify by engine serial
- Mitchell1 ProDemand — 2026 labor and parts cost surveys