Code Details
| Display code | SPN 168 FMI 4 |
|---|---|
| SPN | 168 |
| FMI | 4 |
| OEM code | None listed |
| Manufacturer | Cummins |
| System | Engine �?electrical / power supply |
| Component | Battery / alternator / wiring / ECM power supply |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-12 |
Plain-English Meaning
The ISX15 and X15 ECM continuously monitors battery voltage to ensure the electrical system can power all engine and aftertreatment control circuits. SPN 168 FMI 4 means battery voltage dropped below the minimum threshold. Low voltage can cause a wide range of secondary faults including erratic sensor readings, communication faults, and incomplete regen cycles �?all of which may clear once the charging system is corrected. This is a common fault on trucks with aging batteries, marginal alternators, or high-resistance battery cable connections.
SPN 168 FMI 4 is Battery Voltage below normal operating range. On Cummins CM2350 ISX15/X15, the ECM monitors supply voltage continuously. Insite displays live battery voltage alongside engine speed and load, making it easy to check whether voltage drops under load (alternator issue) or is low at rest (battery issue). FMI 4 indicates a valid reading below the calibration floor, not a wiring fault.
Common Symptoms
- Battery voltage warning on the instrument cluster
- Multiple other fault codes from various modules that rely on stable voltage
- Hard starting �?particularly in cold weather with a weak battery
- Erratic gauge behavior or communication faults alongside low voltage
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Battery weak or sulfated �?cannot hold adequate voltage under load
- Alternator not charging properly (failing regulator, worn brushes, failed diode)
- High-resistance battery cable connections or corroded terminal clamps
- Excessive parasitic draw running the battery down during parking
- Battery voltage sensor fault in the ECM circuit
First Checks
- Load-test each battery �?a battery that reads 12.6 V at rest may drop severely under cranking or load
- Measure alternator output voltage with engine running at 1,200�?,500 RPM �?should be 13.5�?4.5 V
- Check voltage drop across each battery cable under load �?more than 0.2 V indicates high resistance
- Inspect battery terminal clamps for corrosion and confirm cables are tight
- Connect Cummins Insite and monitor live battery voltage under cranking and running conditions
Can I Keep Driving?
Low battery voltage causes unreliable ECM operation and may result in multiple secondary fault codes. Correct the charging system before addressing other faults.
Related Codes
Related Lookup Pages
Sources
- Cummins INSITE Service Tool — Public Reference Documentation Cummins Inc. · oem · accessed 2026-06-11 · confidence medium
Source: Cummins Inc., Cummins INSITE Service Tool — Public Reference Documentation. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - QuickServe Online Cummins Inc. · oem · accessed 2026-05-05 · confidence medium
Source: Cummins Inc., QuickServe Online. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
Can low battery voltage cause the Cummins ISX15 to set false aftertreatment faults?
Yes. Low voltage can cause sensor circuits to read incorrectly, communication faults between modules, and incomplete regen cycles �?all of which generate fault codes. When SPN 168 FMI 4 appears alongside multiple other faults, correct the battery and charging system first. Many secondary codes will clear on their own once voltage is restored to normal.
Does the Cummins ISX15 have dual batteries, and how do I test them?
Most Class 8 trucks with the ISX15 have dual 12V batteries wired in parallel to provide adequate cranking amperage. Each battery should be load-tested individually. If one battery is weak, the healthy battery can mask the problem at rest but both will show low voltage under cranking load. Both batteries in a dual-battery system should be replaced as a pair when one fails.
What is the normal operating voltage range for the Cummins ISX15 ECM?
The Cummins ISX15 and X15 ECM is designed to operate in the 11�?5 V range (12V nominal system). SPN 168 FMI 4 triggers when voltage drops below the calibration floor �?typically around 11 V. Sustained operation below 11 V risks erratic ECM behavior and data communication failures. Insite displays the live voltage reading for direct comparison.