What causes a SPN 7840 code?

SPN 7840 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 SPN 7840 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 7840 active.

How do I fix SPN 7840?

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

J1939 SPN

SPN 7840

Cummins fault code 7840

What it means

Ambient Air Temperature - Data Erratic, Intermittent, or Incorrect. The ECM has detected that the ambient air temperature sensor reading is not changing in conjunction with other engine parameters.