What causes a SPN 303 code?

SPN 303 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 303 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 303 active.

How do I fix SPN 303?

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 303

J1939 SPN 303 — Fuel/Air parameter

What it means

SPN 303 indicates an Air Filter Differential Pressure or more broadly a 'Fuel/Air' ratio mismatch where the engine detects insufficient air volume for the commanded fuel.