What causes a P1138 code?

P1138 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 P1138 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P1138 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

P1138

WRAF heater CJ125 undervoltage fault

What it means

The P1138 code indicates that the CJ125 driver chip (which controls the oxygen/NOx sensor) has detected an undervoltage condition on the sensor's heater circuit, preventing the sensor from reaching its required operating temperature.