Failure mode - Ford 6.7L Power Stroke Diesel (Scorpion)

DEF / SCR / NOx sensor faults

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

Symptoms

check-engine light with codes P20EE (SCR efficiency below threshold), P2200-series (NOx sensor circuit/performance), P204F (reductant/DEF system performance)
progressive derate warnings culminating in a 5mph limp if unaddressed
DEF warnings/low-dose faults that persist even with a freshly filled, good-quality tank

Compare with the other causes of the same sign on this engine: loss of power, limp mode or derate (5 causes) · def, dpf and regeneration warnings (2 causes)

Root cause

Every 6.7L carries the full modern aftertreatment suite (diesel oxidation catalyst, DPF, SCR with DEF dosing to convert NOx to nitrogen and water). Per an aftermarket parts maker's 6.7L Power Stroke guide, aftertreatment faults are the most common check-engine-light source on gen3 (2020+) trucks specifically, and among the most misdiagnosed: a P20EE efficiency code can come from a genuinely lazy NOx sensor (one upstream, one downstream of the SCR catalyst), a crystallized/restricted DEF injector causing real underdosing that MIMICS a bad sensor, contaminated DEF, an exhaust leak upstream of the sensors, or DEF quality/level sensors inside the tank failing outright - replacing a sensor on a system that is genuinely underdosing fixes nothing and wastes a non-cheap part.

Diagnosis

verify actual dosing BEFORE replacing any sensor: test DEF concentration with a refractometer, inspect the DEF injector tip for crystal buildup, confirm commanded dosing with a scan tool, check for exhaust leaks upstream of the sensors
NOx sensors are not cheap - do not condemn one without first ruling out the injector/DEF-quality/leak alternatives above

Fixes

replace a confirmed-failed NOx sensor (upstream or downstream of the SCR catalyst) only after verifying dosing ()

nox-sensor - more than one option: NOx Sensor, Upstream/Front, NOx Sensor, Upstream/Front (current service PN DC3Z-9D378-D), NOx Sensor, Downstream/Rear, NOx Sensor, Downstream/Rear (current service PN DC3Z-9D378-C)

replace a crystallized/restricted DEF injector ()

def-injector - more than one option: DEF (Diesel Exhaust Fluid) Injector, DEF (Diesel Exhaust Fluid) Injector (current service PN AL3Z-5J281-B)

replace the DEF quality/level sensor or DEF pump/heater for tank-side faults, especially in cold climates where the system must thaw frozen fluid ()

def-tank-sensor - more than one option: 6.7L Power Stroke DEF Reservoir Assembly (Ford BC3Z-5J225-K, 2011-2012), 6.7L Power Stroke DEF Quality/Level Sender (Ford FC4Z-5J229-A, 2017)
DEF Pump/Sender Module (Emission Reduction System)

Prevalence

very-common

Parts for this repair run about $110-$4,400: see them next to the other repairs on this engine in repair costs.