What causes a SPN 399 code?

SPN 399 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 399 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 399 active.

How do I fix SPN 399?

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 399

J1939 SPN 399 — Fuel/Air parameter

What it means

The J1939 SPN 399 code is a generic parameter identifier for Fuel/Air feedback. It often indicates that the ECM cannot calculate or broadcast a valid fuel/air ratio or fuel rate to the data link.