What causes a P1840 code?

P1840 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 P1840 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P1840 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

P1840

Upstream NOx sensor AT1IGC1 low rate fault

What it means

The Upstream (Inlet) NOx sensor is reporting a 'slow response' or rate of change fault, meaning its signal isn't fluctuating as expected during engine load changes.