What causes a P1859 code?

P1859 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 P1859 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P1859 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

P1859

Upstream NOx sensor O2 persistent invalid monitor fault

What it means

The engine control module (ECM) has detected that the upstream (pre-SCR) NOx sensor's oxygen concentration signal is not changing as expected or is providing a value that is physically impossible given current engine operating conditions.