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SPN 168 — Battery voltage

Safe to driveTo the repair shop

Updated: 2026-11-14

J1939 code SPN 168. Below — how the fault is worded at the factory by Cummins ISB, DAF, Detroit Diesel, Dongfeng, International, KAMAZ, KAMAZ-5490, MAZ, Mercedes-Benz, Renault Trucks, SITRAK and YaMZ, and what the second half of the code (FMI) means in the standard.

How to check it yourself

The ECU measures the voltage at its own power input. This code means the ECU sees the wrong voltage: the cause is in the batteries, the alternator, or the wires and grounds between them and the ECU.

What it should read

Two charged 12 V batteries, engine offvoltage normal for a 24 V on-board system
Engine running, alternator chargingvoltage higher than with the engine off, but normal for a 24 V system
Battery terminals and the ECU input under loadnearly the same voltage, no noticeable drop

Check order

  1. Measure at the battery terminals with the engine off, then with it running.
  2. Voltage above normal with the engine running — alternator regulator. Below normal — alternator, belt, or the wire from the alternator.
  3. Battery voltage is normal but the ECU sees less: measure at the ECU power connector with a load on (headlights, heater fan). A noticeable drop means poor contact: the battery terminals, the ECU ground, the engine-to-frame ground, the power fuse.
  4. The code appears only when cranking — check the batteries with a load tester or meter: a sagging battery drops the voltage and the ECU restarts.
  5. If several ECUs have this code at once, the cause is common: batteries, alternator or the main ground, not the ECU.

Common causes — from most to least frequent

  1. Corroded or loosely tightened battery terminals.
  2. Engine-to-frame ground or ECU ground.
  3. Worn or discharged batteries.
  4. Alternator regulator, brushes, belt.
  5. ECU power fuse or relay with burnt contacts.

Cummins ISBDAFDetroit DieselDongfengInternationalKAMAZKAMAZ-5490MAZMercedes-BenzRenault TrucksSITRAKYaMZ

Cummins ISB 6.7 engine

FMI 15Battery 1 voltage high, least severe
FMI 16Battery 1 voltage high, moderately severe
FMI 17Battery 1 voltage low, least severe
FMI 18Battery 1 voltage low, moderately severe

DAF

FMI 16ECU power supply, pin C4 voltage too high (>36V)
FMI 18ECU power supply, pin C4 voltage too low (<6V)

Detroit Diesel

FMI 0Battery voltage high. Check the alternator.
FMI 1Battery voltage below normal. Check the battery terminals and grounds, and the alternator belt.
FMI 14DD15: Optimized Idle – Charging System or Battery Failure; DD8: ECU Powerdown Not Completed (Main Battery Terminal Possibly Floating)

Dongfeng

FMI 16Battery voltage too high
FMI 18Battery voltage too low

International

FMI 3Electrical Potential (Voltage) — DC Module Overvoltage condition on Vehicle DC Bus.; Electrical Potential (Voltage) — AC Module Overvoltage condition on High Voltage DC Bus.; Electrical Potential (Voltage) — Battery Voltage High
FMI 4Electrical Potential (Voltage) — DC Module Undervoltage condition on Vehicle DC Bus.; Electrical Potential (Voltage) — AC Module Undervoltage condition on High Voltage DC Bus.; Electrical Potential (Voltage) — Battery Voltage Low
FMI 6Electrical Potential (Voltage) — AC Module has shutdown due to overload condition.
FMI 16Electrical Potential (Voltage) — DC module over temperature condition.; Electrical Potential (Voltage) — AC module over temperature condition.

KAMAZ engine (J1939)

FMI 3Battery supply voltage out of range.
FMI 4Battery supply voltage out of range.

KAMAZ-5490 (Mercedes, K-line)

FMI is per the block maker's own table, not the J1939 table below.

FMI 3On-board supply voltage: voltage high. K-line code on KamAZ-5490: MR2 17542.
FMI 4On-board supply voltage: voltage low. K-line code on KamAZ-5490: MR2 17543.

MAZ

FMI 3Battery supply voltage out of range - Battery voltage high. Repair: Check the battery and its wiring.
FMI 4Battery supply voltage out of range - Battery voltage low. Repair: Check the battery and its wiring.

Mercedes-Benz

FMI 1Battery Voltage Very Low
FMI 7Opt Idle Detected Charging System or Battery Failure
FMI 9Main battery connection lost

Renault Trucks

FMI 0Battery voltage measured by the EECU processor too high. Repair: The alternator regulator is faulty.
FMI 1Battery voltage measured by the EECU processor too low. Repair: The alternator or the batteries are faulty.
FMI 3Battery voltage measured by the control unit above 40 V. Repair: If the voltage is normal, replace the EECU.
FMI 4Battery voltage measured by the control unit below 5.5 V. Repair: If the voltage is normal, replace the EECU.
FMI 12Battery voltage reading fault. Repair: Replace the EECU.
FMI 15Battery voltage measured by the EECU processor too high. Repair: The alternator regulator is faulty.
FMI 17Battery voltage measured by the EECU processor too low. Repair: The alternator or the batteries are faulty.

SITRAK

FMI 3System voltage above the upper limit for the actuator supply. Scan tool code: P0563. Effect: OBD limits torque immediately.
FMI 4System voltage below the lower limit for the actuator supply. Scan tool code: P0562.
FMI 15High battery voltage
FMI 17Low battery voltage

YaMZ

FMI 3Battery supply voltage out of range (Battery voltage high). Repair: Check the battery and its wiring.
FMI 4Battery supply voltage out of range (Battery voltage low). Repair: Check the battery and its wiring.

What FMI means in J1939

SPN 168 FMI 0168/0 · 168.0data above normal range (most severe level)
SPN 168 FMI 1168/1 · 168.1data below normal range (most severe level)
SPN 168 FMI 3168/3 · 168.3voltage above normal or shorted high
SPN 168 FMI 4168/4 · 168.4voltage below normal or shorted to ground
SPN 168 FMI 6168/6 · 168.6current above normal or grounded circuit
SPN 168 FMI 7168/7 · 168.7mechanical system not responding
SPN 168 FMI 9168/9 · 168.9abnormal update rate
SPN 168 FMI 12168/12 · 168.12faulty device or component
SPN 168 FMI 14168/14 · 168.14manufacturer-specific instructions
SPN 168 FMI 15168/15 · 168.15above normal — low severity
SPN 168 FMI 16168/16 · 168.16above normal — moderate severity
SPN 168 FMI 17168/17 · 168.17below normal — low severity
SPN 168 FMI 18168/18 · 168.18below normal — moderate severity

Can I keep driving

You can drive to the nearest repair shop, but do not put it off — if only the amber lamp is on and power is normal. A red lamp, a drop in power or trouble starting — stop.

What happens if you keep driving

Time you have: To the repair shop

The supply voltage the ECU sees is out of range — high or low, depending on the FMI. While only the amber lamp is on and power is normal, the engine usually keeps running.

The cause is most often the alternator and its belt, the terminals and grounds, or weak batteries. Do not put it off: if the supply keeps sagging, some functions may be limited.

In J1939 FMI 0 and 1 are the most severe level; for FMI 17 and 18 the sources describe an amber lamp. If in doubt, it is safer to stop and look on the spot.

What to check on the spot

Check the battery terminals and the ground — corrosion and loose terminals scatter false codes across the whole truck. Then measure the voltage with the engine off and running: with the engine running the alternator should be charging, and the voltage should stay within the normal range of the 24 V on-board system.

Nearby: power supply

Related codes

Makes with this code

KAMAZMAZYaMZSITRAKDongfengDAFMercedes-BenzRenault TrucksCummins ISBDetroit DieselInternationalKAMAZ-5490

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