What causes a SPN 420 code?

SPN 420 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 420 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 420 active.

How do I fix SPN 420?

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 420

J1939 SPN 420 — Fuel/Air parameter

What it means

J1939 SPN 420 is a generic parameter for 'Fuel/Air Ratio' or 'Fuel/Air Control.' It indicates that the calculated ratio of air to fuel is outside of the expected operational window.