Code Details
| Display code | SPN 168 FMI 4 |
|---|---|
| SPN | 168 |
| FMI | 4 |
| OEM code | None listed |
| Manufacturer | Detroit Diesel |
| 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 Detroit DD13 ECM continuously monitors supply voltage to ensure all engine and aftertreatment control circuits have adequate power. SPN 168 FMI 4 means battery voltage dropped below the minimum threshold the ECM expects. Low voltage can cause a cascade of secondary fault codes from sensors and modules that rely on stable power — erratic readings, communication timeouts, and incomplete aftertreatment regen cycles can all appear alongside this code. Correcting the battery or charging system issue usually clears many of these companion faults.
SPN 168 FMI 4 is Battery Potential below normal operating range. On Detroit GHG17 DD13 engines, the MCM (Motor Control Module) monitors battery voltage continuously and shares this data with other modules over the J1939 datalink. FMI 4 indicates a valid reading below the calibration floor — this is not a wiring open or short, but a genuinely low voltage condition. DiagnosticLink can display live battery voltage, making it straightforward to observe whether voltage drops under load (alternator output issue) versus at rest (battery capacity issue).
Common Symptoms
- Battery voltage warning on the instrument cluster or Virtual Technician alert
- Multiple secondary fault codes from ECM, ACM2, or other modules — many may be voltage-related
- Hard starting in cold weather with a weak battery
- Erratic sensor readings or communication errors on the J1939 datalink alongside the low voltage fault
- Incomplete aftertreatment regen cycles if voltage is too low during regen attempts
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Weak or sulfated battery that cannot hold adequate voltage under load
- Alternator not charging properly due to a failing regulator, worn brushes, or failed diode
- High-resistance battery cable connections or corroded terminal clamps
- Excessive parasitic draw depleting the battery during parking periods
- Battery voltage sensor circuit fault in the ECM power supply path
First Checks
- Load-test each battery individually — a battery that reads 12.6 V at rest can drop significantly under cranking or accessory load
- Measure alternator output voltage at 1,200 to 1,500 RPM — normal range is 13.5 to 14.5 V
- Check voltage drop across each battery cable under load — more than 0.2 V indicates high resistance in the cable or terminal
- Inspect all battery terminal clamps for green or white corrosion and confirm each is tight
- Connect Detroit DiagnosticLink and monitor live battery voltage under cranking and running conditions to identify when and how severely the voltage drops
Can I Keep Driving?
Low battery voltage causes unreliable ECM operation and can generate multiple secondary fault codes. Correct the battery and charging system before investigating other fault codes — many companion faults will clear once voltage is stable.
Related Lookup Pages
Sources
- Detroit Diesel Service and Diagnostic Resources — Public Reference Detroit Diesel (Daimler Truck North America) · oem · accessed 2026-06-11 · confidence medium
Source: Detroit Diesel (Daimler Truck North America), Detroit Diesel Service and Diagnostic Resources — Public Reference. 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 Detroit DD13 to set false aftertreatment or sensor faults?
Yes. When battery voltage drops below the ECM's minimum operating threshold, sensor reference voltages become unreliable, J1939 communication can fault, and the aftertreatment control module may log timeout or calibration errors. It is common to see five to ten or more fault codes appear alongside SPN 168 FMI 4. Correct the battery and charging system first — the majority of secondary faults on a DD13 with voltage issues will clear once the electrical system is restored to normal.
Does the Detroit DD13 have dual batteries, and how should I test them?
Most Class 8 trucks with the DD13 use dual 12V batteries wired in parallel to provide sufficient cranking amperage for cold starting. Each battery should be tested individually with a carbon pile load tester or conductance tester. A healthy battery at rest may appear fine but one or both batteries can fail under cranking load. Both batteries in a series or parallel system should be replaced as a set when one fails to avoid a repeat failure.
What is the minimum voltage the Detroit DD13 ECM requires to operate normally?
The Detroit DD13 MCM and associated modules are designed to operate on a 12V nominal system, with normal running voltage between approximately 13.5 and 14.5 V. SPN 168 FMI 4 is set when voltage drops below the calibration floor, typically around 11 V or less. At voltages in this range, the MCM may behave erratically and the J1939 datalink can experience communication faults. DiagnosticLink displays live battery voltage for direct observation.