Code Details
| Display code | SPN 168 FMI 4 |
|---|---|
| SPN | 168 |
| FMI | 4 |
| OEM code | None listed |
| Manufacturer | Volvo |
| 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 Volvo D13 ECU monitors battery supply voltage to ensure all engine and aftertreatment modules have reliable power. When voltage drops below the minimum threshold, SPN 168 FMI 4 is set and a range of secondary faults can appear from sensors and modules affected by the low voltage. In Volvo VNL and VNR trucks, this fault is most commonly caused by aging batteries, a failing alternator, or deteriorated battery cable connections. Fixing the electrical system resolves both this fault and most companion codes.
SPN 168 FMI 4 is Battery Potential below normal range in J1939. On Volvo D13 engines, the ECU monitors supply voltage continuously and shares the reading with the aftertreatment system and other control modules over the SAE J1939 datalink. FMI 4 indicates the measured value is below the calibration floor — a valid low reading, not a sensor fault. VCADS Pro or the Volvo Tech Tool displays the live battery voltage along with other electrical system parameters.
Common Symptoms
- Battery or charging system warning lamp on the Volvo instrument cluster
- Volvo Vehicle Management System alert for low electrical system voltage
- Multiple secondary fault codes from various control modules caused by unstable voltage
- Possible hard starting in cold conditions with a weak battery
- Erratic readings from sensors or intermittent J1939 communication timeouts
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Weak or sulfated battery no longer holding adequate charge
- Alternator output insufficient — internal fault in regulator, rectifier, or stator
- High-resistance battery cable or ground connection reducing effective voltage at the ECU
- Parasitic draw from accessory equipment or a module fault depleting batteries during parking
- Battery or ECU power supply circuit fault
First Checks
- Perform a load test on each battery — a static open-circuit voltage test is not sufficient for identifying a failed battery
- Measure alternator charging voltage at approximately 1,200 to 1,500 RPM — expected range is 13.5 to 14.5 V
- Check voltage drop across battery cables and ground straps under load
- Inspect battery terminals for corrosion and confirm all connections are clean and tight
- Connect VCADS Pro or Volvo Tech Tool and monitor live battery voltage under cranking and running conditions
Can I Keep Driving?
Low battery voltage on the D13 causes unreliable ECU operation and a wave of secondary fault codes. Address the battery and charging system before troubleshooting other faults — most voltage-related secondary codes clear once stable voltage is restored.
Related Lookup Pages
Sources
- Volvo Trucks Tech Info Public Resources Volvo Trucks North America · oem · accessed 2026-06-11 · confidence medium
Source: Volvo Trucks North America, Volvo Trucks Tech Info Public Resources. 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 the Volvo D13 log SPN 168 FMI 4 even when the truck appears to start and run normally?
Yes. The ECU can detect voltage drops that are brief or occur only under high electrical load — for example, during a parked regen, when multiple accessories are active, or during a cold start. These transient low-voltage events may not be obvious to the driver but are logged by the ECU. VCADS Pro can review the fault occurrence history to see under what conditions the code was set.
Should I replace both batteries if only one is failing on the Volvo D13?
Replacing batteries as a matched pair is strongly recommended in dual-battery systems. When one battery in a parallel system weakens, the remaining battery must work harder to compensate, which accelerates its own degradation. Installing a new battery alongside an aging one often leads to a repeat failure within a short period. Replacing both batteries at the same time is the more economical long-term approach.
Does Volvo have a specific alternator output specification for the D13 to prevent SPN 168 FMI 4?
Volvo specifies alternator output voltage requirements in the D13 service documentation. Normal charging system voltage on a Volvo VNL or VNR should be in the 13.5 to 14.4 V range at the batteries when the engine is running at moderate speed. VCADS Pro or Tech Tool can monitor live charging system data. If the alternator consistently produces below 13.5 V under normal load, the alternator output is insufficient and the unit should be tested further or replaced.