SPN 627 FMI 2 — ECU Power Supply — Intermittent / Erratic
The engine ECU is seeing erratic or intermittent supply power. Voltage drops, bad grounds, or a failing relay can make the ECU reset or behave unpredictably — sometimes causing stalls or no-starts. Often paired with SPN 629 (controller internal) faults.
Your SPN/FMI rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Serious. Limp to the next safe stop.
Most likely causes
- Corroded or loose battery terminals, grounds, or ECU power connector
- Failing main power relay or blown/intermittent fuse
- Low battery voltage or a charging-system (alternator) problem
- Chafed power/ground harness causing intermittent open
First moves (roadside)
- Check and clean battery terminals and main grounds — a bad ground mimics dozens of faults.
- Measure battery and charging voltage; weak batteries/alternator cause erratic ECU power.
- Wiggle-test the ECU power/ground connector while watching for the fault to flicker.
- Inspect the main power relay and associated fuses before condemning the ECU.
Common misdiagnoses
- Replacing the ECU when the real fault was a corroded ground or loose battery terminal.
- Chasing engine codes that are really just symptoms of dirty ECU power.
- Overlooking a weak alternator that drops voltage under load.
FAQ
The engine ECU is seeing erratic or intermittent supply power. Voltage drops, bad grounds, or a failing relay can make the ECU reset or behave unpredictably — sometimes causing stalls or no-starts. Often paired with SPN 629 (controller internal) faults.
Serious — Limp to the next safe stop.
The most common cause is: Corroded or loose battery terminals, grounds, or ECU power connector. Other possibilities include Failing main power relay or blown/intermittent fuse; Low battery voltage or a charging-system (alternator) problem; Chafed power/ground harness causing intermittent open.
Limp to the next safe stop. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.