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

EGR carbon buildup (valve and intake deposits)

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

Symptoms

gradual rough idle / drivability degradation over high mileage rather than sudden failure
false knock-sensor codes from soot fouling in the EGR path are reported by shops as a secondary symptom

Compare with the other causes of the same sign on this engine: misfire, rough idle, hesitation or surging (4 causes)

Root cause

Recirculated exhaust mixes with oil vapor and deposits carbon on the EGR valve and related components - vendor framing treats this as normal wear on the L5P, not a defect. Unlike the 6.7 Power Stroke and the 2010+ Cummins, the EGR cooler itself is not a loud community failure point on the L5P; NHTSA VOQ is nearly silent on EGR (1 word-boundary hit in 1202 complaints).

Diagnosis

inspect the EGR valve and intake for carbon at service intervals; clean rather than replace when the valve still actuates
before chasing knock-sensor codes on a high-miler, check for EGR soot fouling the sensor circuits

Fixes

clean or replace the EGR valve and clean intake deposits; replace the EGR cooler if coolant-side symptoms appear (see diagnosis) ()

Prevalence

common