SPN 4364Aftertreatment 1 SCR conversion efficiency

Means the aftertreatment control module (ACM)—working with engine ECM data—detected SCR NOx conversion efficiency below expected on aftertreatment system 1. Industry diagnostics in the Detroit/DTNA style treat this as an inlet-versus-outlet NOx problem: the controller compares SCR inlet NOx (commonly framed with SPN 3216) to SCR outlet NOx (commonly framed with SPN 3226) under dosing-enabled conditions and flags efficiency when the outlet does not fall enough relative to the inlet. It is the heavy-duty efficiency companion to OBD codes such as P20EE. Drivers may see DEF/SCR warning lamps and rising DEF consumption anomalies long before a hard derate; ignored efficiency faults escalate toward operator inducement (often SPN 5246) with progressive speed and torque limits.

Enabling and verification matter as much as the parts list. Efficiency logic typically needs DEF dosing enabled, meaningful engine load, and SCR brick temperature inside the OEM window—idle shop bay tests with a cold mixer waste time. Many OEM procedures verify conversion with a parked regeneration or a loaded road test that holds the SCR in-range while dosing quantity and NOx rationality are watched live. Do not clear SPN 4364 and call it fixed until you can show inlet NOx dropping across the brick with dosing confirmed.

Rank causes by shop frequency, not sticker price. Most often: contaminated, diluted, or wrong-concentration Diesel Exhaust Fluid (DEF / urea ~32.5%). A handheld refractometer should read roughly 31–34% urea; off-spec fluid, water dilution, or gasoline/DEF mix-ups destroy conversion without hurting a new SCR brick. Very common next: doser or mixer crystallization that blocks spray and ammonia distribution. Common: biased inlet or outlet NOx sensors that invent a false efficiency gap, and duty cycles that never hold SCR temperature (short vocational runs, excessive idle). Aged SCR catalyst brick belongs last—condemn it only after DEF quality, dosing path, exhaust temperature, and both NOx sensors prove clean. Always clear air-path, EGR, fueling, and NOx-sensor circuit faults before you authorize an SCR can.

Step-through: pull ACM/ECM freeze data and related SPNs; refractometer-test DEF and replace the tank contents if concentration is wrong; inspect decomposition tube/mixer for crystals and run OEM doser quantity tests hot; compare SPN 3216 vs 3226 (or equivalent PIDs) during a parked regen or loaded SCR window; validate heater/power/ground on both NOx sensors before swapping bricks. Replace the SCR catalyst only when dosing quantity and quality are proven and outlet NOx still tracks inlet. Dig elsewhere when efficiency “fails” only cold, when one NOx PID is stuck or irrational, or when EGR/NOx circuit codes are still active. System context: this is DEF/NOx/ACM aftertreatment chemistry—not a gasoline O2 catalyst judgment. Persistent 4364 without repair is how fleets walk into inducement.