Failure mode - Duramax 6.6L V8 Turbo Diesel (GM HD, L5P 2017+ generation)

NOx sensor drift/failure (SCR efficiency codes)

Reliability tier: verified - verify before ordering, see /disclosure/

Symptoms

check-engine light with SCR/NOx efficiency codes ('service emissions system')
downstream NOx reading drifting toward the upstream value (a healthy SCR shows roughly a 70-90% reduction)

Compare with the other causes of the same sign on this engine: def, dpf and regeneration warnings (3 causes)

Root cause

The two NOx sensors (before/after the SCR catalyst) drift and fail, commonly in the 100k-150k mile range on L5P. The sensor is not always guilty: DEF quality issues and SCR system faults can trigger the same codes.

Diagnosis

compare live upstream vs downstream NOx readings with a scan tool before replacing the sensor
rule out DEF quality and dosing faults first - the codes are shared

Fixes

replace the failed NOx sensor (upstream or downstream) after live-data diagnosis ()

nox-sensor - more than one option: 6.6L Duramax L5P NOx Sensor, Position 2 / Downstream 2017-2019 (GM 12741484), 6.6L Duramax L5P NOx Sensor, Position 2 / Downstream 2020-2023 (GM 12747870), 6.6L Duramax L5P NOx Sensor, Position 1 / Upstream (GM 12718742)

Prevalence

common