Code Details
| Display code | SPN 168 FMI 4 |
|---|---|
| SPN | 168 |
| FMI | 4 |
| OEM code | None listed |
| Manufacturer | Mack |
| 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 Mack MP8 ECU monitors electrical system voltage to ensure stable operation of all engine and aftertreatment control circuits. Low battery voltage produces unreliable sensor readings, module communication errors, and incomplete aftertreatment operations — all of which can generate additional fault codes. This is a common fault on trucks with aging batteries, overloaded alternators, or high-resistance cable connections. Correcting the electrical system root cause resolves this fault and typically clears most companion codes.
SPN 168 FMI 4 is Battery Potential below normal operating range in J1939. On Mack MP8 engines — which share the Volvo Group D13 platform — the ECU monitors battery voltage continuously. FMI 4 means the measured voltage is below the calibration threshold for normal operation. This is a valid below-normal reading, not a wiring open or short circuit. Mack PTT provides a live battery voltage display and the ability to check other electrical system parameters during diagnosis.
Common Symptoms
- Battery or charging warning lamp on the Mack instrument cluster or GuardDog Connect alert
- Multiple fault codes from engine, aftertreatment, or body control modules associated with voltage instability
- Difficult cold starting with a weak battery
- Intermittent J1939 communication timeouts or module communication faults
- Erratic DEF dosing or aftertreatment issues during periods of low voltage
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Battery capacity degraded — no longer holding adequate voltage under load
- Alternator failure — regulator, rectifier, or brush pack fault reducing output
- High-resistance battery cables, terminals, or ground connections
- Parasitic drain from an accessory or module keeping power on during parking
- Battery isolation circuit or ECU power supply fault
First Checks
- Load-test each battery individually using a carbon pile tester or conductance analyzer
- Check alternator charging voltage with the engine running at moderate speed and all normal electrical loads on
- Measure voltage drop across the positive battery cables and ground straps under load
- Inspect all battery terminal connections and engine ground straps for corrosion or looseness
- Connect Mack PTT and monitor live battery voltage under cranking and at operating speed to pinpoint when and how the voltage drops
Can I Keep Driving?
Low voltage on the MP8 causes unreliable ECU operation and multiple secondary fault codes. The battery and charging system must be corrected before diagnosing other faults — many companion codes will resolve once stable voltage is restored.
Related Lookup Pages
Sources
- Mack Trucks Service Support Public Resources Mack Trucks Inc. · oem · accessed 2026-06-11 · confidence medium
Source: Mack Trucks Inc., Mack Trucks Service Support 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 I use a generic battery tester to diagnose SPN 168 FMI 4 on the Mack MP8?
A quality battery conductance or carbon pile load tester is suitable for testing the batteries themselves. Alternator output can be measured with a digital multimeter at the battery posts. Mack PTT is needed to confirm the ECU battery voltage reading, check for companion codes, and verify that no internal module fault is contributing to the low voltage condition. Generic OBD tools may show the fault code but lack access to live voltage data within the ECU's parameters.
Does the Mack MP8 use the same battery specifications as the Volvo D13?
The Mack MP8 and Volvo D13 share the same Volvo Group powertrain platform, and the electrical system specifications are similar. Class 8 trucks with the MP8 typically use two group 31 or group 4D batteries in parallel. Specific cold cranking amperage requirements depend on the engine displacement and the climate in which the truck operates. Always consult the Mack chassis specifications for the correct battery group and CCA rating.
If the Mack MP8 battery voltage appears normal at idle, can the alternator still be the problem?
Yes. Alternator problems can be intermittent or load-dependent. A diode failure in the alternator may produce adequate voltage at light load but drop below spec when the accessory load increases — including HVAC, lights, and aftertreatment systems running simultaneously. Measuring alternator output voltage with all normal electrical loads active at 1,200 to 1,500 RPM gives a more representative picture of alternator health than an idle-only test.