What causes a SPN 391 code?

SPN 391 can be triggered by several issues — see the verified probable causes and step-by-step diagnostics shared by mechanics and owners on OBDIY.

Can I drive with a SPN 391 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 391 active.

How do I fix SPN 391?

Follow the step-by-step diagnostic on this page, then read the verified fixes contributed by other Heavy Truck owners — the answer marked "Verified Cause" worked for the original poster.

OBDIY

J1939 SPN

SPN 391

J1939 SPN 391 — Fuel/Air parameter

What it means

SPN 391 indicates a fault with the Fuel/Air Ratio Control (FARC) system, which manages the fuel-to-air mixture to prevent excessive smoke during acceleration.