Failure mode - GM 3.0L Duramax LM2/LZ0 I6 (2020+)

DEF system faults: tank heater, pump/quality (speed-limited derate)

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

Symptoms

'Exhaust Fluid System' / DEF quality warnings, countdown to speed limitation
hard 5-mph derate when ignored; NOx codes accompany tank-side heater/pump faults

Compare with the other causes of the same sign on this engine: loss of power, limp mode or derate (7 causes) · def, dpf and regeneration warnings (4 causes)

Root cause

DEF tank-side hardware (heater, pump/delivery module, quality sensor) faults trigger the SCR derate ladder. 50 of 1,190 VIN-confirmed NHTSA complaints, spread across all model years - the standard DEF-era pattern shared with the HD trucks.

Diagnosis

read the DEF/NOx codes together: tank-side heater/pump codes before sensor codes
verify DEF quality/fill before replacing hardware (contaminated or frozen fluid mimics hardware faults)

Fixes

replace the DEF tank heater / pump-delivery module when codes point at the tank side ()

def-pump-module - more than one option: 3.0L Duramax DEF Controller (GM 85514348), 3.0L Duramax DEF Tank Assembly, pickups 2020-2022 (GM 86790695), 3.0L Duramax DEF Tank Assembly, pickups 2022-2024 (GM 87867197), 3.0L Duramax DEF Tank Assembly, SUVs 2021-2023 (GM 86772598)

drain and refill with fresh API-certified DEF (service item) ()

DEF refill - fresh API-certified fluid (service item, all 3.0L Duramax)

Prevalence

common

Parts for this repair run about $110-$500: see them next to the other repairs on this engine in repair costs.