What causes a SPN 5166 code?

SPN 5166 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 5166 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 5166 active.

How do I fix SPN 5166?

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 5166

Cummins fault code 5166

What it means

Aftertreatment 1 Diesel Exhaust Fluid Quality - Abnormal Update Rate. This indicates the Aftertreatment Control Module (ACM) has lost communication with the DEF quality sensor or is receiving invalid data.