What causes a SPN 309 code?

SPN 309 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 309 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 309 active.

How do I fix SPN 309?

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 309

J1939 SPN 309 — Fuel/Air parameter

What it means

SPN 309 indicates that the engine controller has detected an irrational or out-of-range value regarding the Fuel/Air mixture ratio, typically derived from intake manifold pressure and fuel rate calculations.