What causes a SPN 304 code?

SPN 304 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 304 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 304 active.

How do I fix SPN 304?

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 304

J1939 SPN 304 — Fuel/Air parameter

What it means

SPN 304 indicates a Fuel/Air parameter error, specifically relating to the Fuel Injection Pressure system being unable to reach or maintain the commanded setpoint.