What causes a P1861 code?

P1861 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 P1861 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P1861 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

P1861

Upstream NOx sensor O2 signal-related data out of range fault

What it means

The Engine Control Module (ECM) detects that the oxygen concentration reading from the inlet (upstream) NOx sensor is physically impossible or inconsistent with current engine operating conditions (such as during motoring or high-load states).