What causes a SPN 3221 code?

SPN 3221 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 3221 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 3221 active.

How do I fix SPN 3221?

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 3221

SPN 3221 FMI 5/3/4 - Aftertreatment 1 Intake NOx Sensor Circuit

What it means

Indicates a circuit failure or out-of-range signal for the Intake (Pre-SCR) NOx sensor. This sensor is critical for calculating the urea injection rate for Diesel Exhaust Fluid.