Truck Fault Codes

PACCAR MX-13 SPN 168 FMI 4 — Battery Voltage Below Normal

SPN 168 FMI 4 on a PACCAR MX-13 means the engine management system has detected battery voltage below the acceptable operating range. Load-test the batteries, check alternator output, and inspect battery cable connections. PACCAR ESA displays live battery voltage to confirm whether the issue is at rest or under load.

Code Details

Structured details for SPN 168 FMI 4
Display codeSPN 168 FMI 4
SPN168
FMI4
OEM codeNone listed
ManufacturerPACCAR
SystemEngine — electrical / power supply
ComponentBattery / alternator / wiring / ECM power supply
Source addressUnknown or not applicable
Severitymedium
Review statussource backed
Source confidencemedium
Last reviewed2026-06-12

Plain-English Meaning

The PACCAR MX-13 EMS monitors supply voltage to ensure all engine control circuits have sufficient power. Low voltage causes a range of secondary problems — erratic sensor readings, module communication faults, and unreliable aftertreatment operation. When voltage drops below the calibration minimum, SPN 168 FMI 4 is logged. Most cases trace to a weak battery, an underperforming alternator, or high-resistance cable connections. Correcting the charging system typically resolves the direct fault and clears many companion codes.

SPN 168 is Battery Potential in J1939, with FMI 4 indicating a below-normal reading. On PACCAR MX-13 engines, the EMS monitors battery voltage continuously as part of its self-diagnostic routine. FMI 4 means the measured voltage is below the calibrated floor but within a valid electrical range — it is not a sensor open or short circuit. PACCAR ESA provides live battery voltage readings, making it easy to determine if the voltage drop occurs at rest, during cranking, or under load at operating speed.

Common Symptoms

  • Battery or charging system warning on the Kenworth or Peterbilt instrument cluster
  • Multiple secondary fault codes from various control modules indicating voltage-related issues
  • Hard starting, particularly in cold weather
  • Erratic gauge behavior or J1939 communication timeouts alongside the battery voltage fault
  • Incomplete DEF dosing or aftertreatment regen if voltage is insufficient during those operations

Possible Causes

Possible causes may include the items below. The list is not a parts diagnosis.

  • Weak or degraded battery unable to maintain adequate voltage under load
  • Alternator output failure — regulator, brush pack, or diode issues
  • High-resistance battery cable ends or ground connections
  • Parasitic electrical draw running batteries down during extended park time
  • ECM power supply circuit fault

First Checks

  • Load-test each battery individually — a battery that passes a static voltage test can still fail under load
  • Check alternator output voltage at approximately 1,200 RPM — should be between 13.5 and 14.5 V
  • Perform a voltage drop test on each battery cable under load — values above 0.2 V indicate resistance
  • Inspect battery terminal clamps and ground straps for corrosion and confirm all connections are secure
  • Connect PACCAR ESA and monitor live battery voltage at idle and under load to identify when the voltage is dropping

Can I Keep Driving?

Low battery voltage on the MX-13 can cause unreliable ECM operation and a cascade of secondary fault codes. Repair the battery and charging system before addressing other faults — most voltage-related companion codes will clear once normal voltage is restored.

Related Lookup Pages

Sources

  • PACCAR / Kenworth / Peterbilt Service Support — MX Engine Resources PACCAR Inc. · oem · accessed 2026-06-11 · confidence medium

    Source: PACCAR Inc., PACCAR / Kenworth / Peterbilt Service Support — MX Engine 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 SPN 168 FMI 4 on the PACCAR MX-13 clear itself after the battery charges up?

If the low voltage was caused by a temporary drain event — a door left open overnight or a short-duration parasitic draw — the fault may clear on its own once the battery is recharged and voltage returns to normal. If the fault returns after recharging, the battery may be too degraded to hold an adequate charge, or the alternator output is insufficient to keep up with the vehicle load. A load test and alternator check are recommended when the fault recurs.

Is it normal to see many unrelated fault codes alongside SPN 168 FMI 4 on the MX-13?

Yes. When battery voltage drops below the ECM minimum, sensors throughout the engine and aftertreatment system can read incorrectly because their reference voltages are also low. This produces a cluster of fault codes from multiple systems that appear unrelated but are all caused by the same root condition. Addressing the battery and charging system is the correct first step — do not chase individual sensor faults until voltage is stable.

What alternator output voltage should I expect from a healthy PACCAR MX-13 truck?

A healthy 12V charging system on a PACCAR truck should produce approximately 13.8 to 14.4 V at the batteries with the engine running at moderate speed and electrical loads active. Significantly lower readings indicate an underperforming alternator or high-resistance in the charging circuit. Readings consistently above 14.5 V can indicate overcharging, which can damage batteries over time.