What causes a SPN 491 code?

SPN 491 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 491 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 491 active.

How do I fix SPN 491?

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 491

J1939 SPN 491 — Fuel/Air parameter

What it means

Specifically related to the rationality check between fuel demand and air delivery. The ECM detects that the air/fuel ratio is outside of the mapped parameters for current engine load.