What causes a P1518 code?

P1518 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 P1518 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P1518 active.

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

OBD-II

P1518

DEFOI torque reduction fault

What it means

Engine Derate due to DEF-related Torque Reduction Failure. The ECM has detected that the emissions system cannot comply with regulations, triggering a mandatory power reduction.