Cummins SPN 168 FMI 2: Battery Voltage Data Erratic
SPN 168 FMI 2 (FMI 2) — Electrical Potential (Voltage) - Data Erratic, Intermittent, or Incorrect
Quick Answer
SPN 168 FMI 2 means the ECM's supply voltage is fluctuating instead of holding a steady reading. It's a High severity electrical fault that can cause random resets and module faults across the truck, though it won't trigger an emissions derate on its own.
Overview: Cummins SPN 168 FMI 2
ProfessionalDieselRepair.com treats this code as a ground and connection problem until proven otherwise - it's rarely a simple bad battery.
Repair Snapshot
Time
1 - 3 Hours
Difficulty
Intermediate
Parts Cost
$40 - $350
Dealer Labor
$400+
Tools Required
- Cummins INSITE
- Multimeter
- Battery Load Tester
- Snap-on NEXIQ
What SPN 168 FMI 2 Means
SPN 168 FMI 2 means the ECM is seeing battery voltage that's jumping around rather than holding steady - not a dead battery, but a supply that can't be trusted. This is High severity because an unstable ECM power supply can cause random module resets, hard starts, and other electrical gremlins across the truck.
Symptoms Drivers Notice
- Random dash warning lights that come and go with no pattern
- Intermittent no-crank or stalling, especially over rough roads
- Other modules (transmission, ABS) throwing unrelated communication codes
- Voltage gauge flickering at idle
Most Likely Root Causes
Corroded or loose frame/engine ground strap
CommonElectrical fault - a high-resistance ground causes voltage to fluctuate at the ECM even when the battery itself is healthy.
Loose or corroded battery cable terminals
CommonElectrical fault - resistance at the terminal connection produces an unstable supply, especially under vibration.
Weak battery that passes static voltage but fails under load
OccasionalA genuine battery/charging system issue rather than a wiring fault - confirm with a proper load test, not just a voltmeter.
Failing alternator diode producing AC ripple on the DC supply
OccasionalMechanical/electrical charging system fault - a bad diode injects noise that the ECM reads as erratic voltage.
Common Misdiagnoses (Don't Waste Parts)
Replacing both batteries as a first step
Why it happens: Voltage-related codes get blamed on the battery by default.
Do this instead: This is a parts-cannon shortcut - load-test the existing batteries and check ground strap resistance first. Many of these codes clear with a $15 ground cleaning instead of a $300 battery swap.
Replacing the ECM for an intermittent voltage fault
Why it happens: When the fault won't reproduce on the bench, some shops assume the control module itself is defective.
Do this instead: Verify supply voltage stability at the ECM connector under load and vibration first - a genuinely bad ECM for this fault is rare.
Master Tech Commentary
Mechanic's Notes
ProfessionalDieselRepair.com tip: chase grounds before you chase batteries. A corroded frame ground strap produces the exact same erratic voltage signature as a failing battery, and it's a five-minute fix compared to a battery swap that doesn't solve anything.
Step-by-Step Diagnostic Procedure
- Step 1: Pull codes with Cummins INSITE and check if other modules (ABS, transmission) logged voltage or network faults at the same timestamp - a shared power/ground issue affects multiple systems at once.
- Step 2: With the engine off and key on, measure battery voltage directly at the terminals - should read 12.4-12.8V resting. Then measure voltage at the ECM power pin while wiggling the main battery cable and ground strap.
- Step 3: Load-test both batteries individually; a battery that passes a static voltage check can still sag under load and cause an erratic ECM signal.
- Step 4: Inspect and clean the ground strap connections at the frame and engine block - corrosion or a loose ground is one of the most common causes of erratic ECM voltage.
- Step 5: Check the main power relay and fuse box connections for green corrosion or a loose pin, particularly if the truck runs in salt-belt or coastal conditions.
Live Data & Freeze-Frame Tips
Log SPN 168 continuously on INSITE while road-testing over rough pavement - a voltage trace that dips or spikes in sync with bumps points straight to a loose connection rather than a battery or alternator problem.
Safety Warnings
- Disconnect the negative battery terminal before cleaning ground connections.
- Wear eye protection when cleaning corroded battery terminals - acid residue can splash.
When to Limp In vs Tow
Usually not necessary unless the truck is experiencing repeated stalling - if it stalls unpredictably in traffic, have it towed rather than risk a stall at a dangerous location.
Prevention Tips for Fleets & Owner-Operators
- Clean and re-torque ground straps annually, especially on trucks running coastal or salt-belt routes.
- Dielectric-grease battery terminals during PM service.
- Load-test batteries at every seasonal service, not just when they fail to start.
Related Fault Codes
SPN 168 FMI 4
Voltage below normal on the same circuit - typically points to a more severe low-voltage condition like a dying battery or open circuit.
SPN 168 FMI 0
Voltage above normal - usually an overcharging alternator rather than a connection issue.
Frequently Asked Questions
Can I keep driving with this code active?
Yes, but keep an eye on it - erratic ECM voltage can cause other modules to fault or the truck to stall unexpectedly, so get it looked at soon.
Does this trigger a derate?
No, this is not an emissions-related SPN, so it doesn't start an inducement countdown on its own.
How do I reset it without a scanner?
Disconnecting the battery will clear the stored code, but it will return quickly if the ground or connection issue isn't fixed.
Why do other modules throw codes at the same time?
Because unstable supply voltage affects every module on the truck, not just the engine ECM - that's a strong clue it's a shared ground or connection problem.
Is a new battery the fix?
Usually not. Most of these trace to grounds or terminal corrosion. Load-test first before spending money on batteries.
How much does this repair typically cost?
A ground/terminal cleaning is often under $50 in shop time. A full charging system repair with parts runs $150-$350.