Driver guide · National edition
Published October 30, 2026 · TruckRescueNow driver guide bank, stamped October 2026
The engine computer has logged a fault code. The light alone does not grade the fault — the code does, and modern trucks will often show you the code or its description right on the dash display.
One light covers everything from a loose sensor reading to a fault that will derate you in stages (see the DPF derate article in this series)
Write the code down exactly as displayed before anything is cleared or the key is cycled repeatedly
Steady light, truck running normally: a fault to get read promptly, usually drivable to a planned stop. Flashing or red stop-engine light: stop at the first safe place — the computer is grading this one severe. Light plus a symptom (power loss, smoke, temperature, derate warning): treat it as the symptom, not the light.
The color and behavior of the light is the triage: amber check = schedule it; red stop-engine = stop it
A light that arrives with a countdown ("derate in X hours/miles" on the display) is a deadline, not a suggestion
The gauges (temperature, oil pressure, air), the fluid levels when it is safe to check them, and whether anything about the truck's behavior changed with the light.
Many "faults" are sensor or connection faults on a healthy engine — but you cannot know that from the seat
A light right after fueling, a filter change, or a repair points at the work just done — say so when you call
A mobile mechanic with heavy-duty diagnostic software reads the code, grades it, and clears it if it is a transient fault — the read is the product; the code decides repair, drive-and-monitor, or tow.
Aftertreatment and engine diagnostics are a standard roadside tier; DPF-related diagnostics are published at $400–$700 (Albert's Road Service published rates)
Tell the dispatcher the code from the dash if you have it — the code can pre-load the diagnosis before the truck rolls
Do not pay anyone to "just clear it" without the code recorded: a cleared code with no record deletes your evidence and the fault returns unannounced
Treat every amber light as a shop appointment within days, not weeks — the faults that strand trucks almost all introduced themselves as lights first.
The derate ladder is the standing example: warning, power cut, 5-mph limit — each stage was announced
Fleet drivers: report the light the day it appears, in writing, so the event record protects you when it matures