What causes a P1856 code?

P1856 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 P1856 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P1856 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

P1856

Upstream NOx sensor NOx signal-related data not available fault

What it means

The Engine Control Unit (ECU) is not receiving valid NOx concentration data from the upstream (pre-SCR) NOx sensor.