What causes a P1855 code?

P1855 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 P1855 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P1855 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

P1855

Upstream NOx sensor NOx persistent invalid monitor fault

What it means

The Upstream NOx sensor (Inlet) is providing data that the Engine Control Module (ECM) deems 'plausibly incorrect' or persistent in its failure to report stable values during steady-state engine operation.