What causes a SPN 3721 code?

SPN 3721 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 3721 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 3721 active.

How do I fix SPN 3721?

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 3721

J1939 SPN 3721 — Body/Chassis parameter

What it means

DPF Ash Accumulation - Service Limit Exceeded. This code indicates the engine's control module has calculated that the soot/ash load in the Diesel Particulate Filter has reached its end-of-life service threshold based on mileage/runtime.