What causes a P1851 code?

P1851 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 P1851 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P1851 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

P1851

Upstream NOx sensor heater-related data out of range fault

What it means

The engine control module (ECM) monitors the heater circuit of the upstream (inlet) NOx sensor. This code sets when the ECM detects that the heater current or resistance is outside of expected parameters, preventing the sensor from reaching its operating temperature.