SPN 3226Aftertreatment 1 outlet NOx (SCR downstream / tail)

Is the aftertreatment system 1 outlet NOx sensor—downstream of the SCR catalyst, at the tail of the catalyst can. Tailpipe-side NOx is the feedback that proves urea actually converted NOx across the brick. The ACM compares this outlet reading to inlet NOx (SPN 3216 territory) under dosing-enabled conditions; when outlet NOx stays high relative to inlet, efficiency logic (often SPN 4364) fails. When the outlet sensor itself is slow, open, heater-dead, or moisture-shorted, the controller under-rates a healthy SCR and may start efficiency or inducement timers even though dosing and the brick are fine. That diagnostic job is conversion proof—not DEF dosing-table input. Do not treat 3226 as a clone of 3216: different bung, different failure climate, different first moves.

Location drives the failure signature. The outlet bung sits after the catalyst in a cooler, wetter zone. Condensation, road salt, and connector fretting are more common here than soot melt at the hot inlet. Techs find green corrosion and water in the downstream connector after wash-bay or winter duty long before they find a burned tip. Another outlet-specific pattern: a bung leak admitting false air that collapses the reading and invents a “perfect” conversion while the brick is actually weak—or the opposite, an erratic PID that fails rationality. Leave the intake probe alone unless SPN 3216 is also active; chasing the hot upstream sensor for a condensation fault wastes parts.

Shop-ranked causes: (1) failed outlet-side NOx probe after the SCR brick, (2) condensation shorting the cooler downstream connector, (3) heater open circuit on the outlet smart sensor, (4) corroded CAN/power pins from road spray, (5) outlet bung leak admitting false air, (6) rare ACM efficiency-feedback input failure after the outlet path proves clean. Moisture-related intermittents that set only after cool-down or rain are classic 3226—not classic 3216.

Diagnostic emphasis: locate the post-SCR bung at the tail of the can before wrenching; dry and dielectric-grease the downstream connector if moisture is present; measure heater amps and CAN on the outlet module address only; graph outlet vs inlet NOx during a parked regen or loaded SCR temperature window with dosing enabled—healthy conversion shows outlet falling versus inlet; an outlet PID stuck, open, or heater-cold fails first as a sensor path. Replace the outlet-location NOx sensor only when wiring and connector dryness check out yet the PID stays irrational. Dig elsewhere when inlet dosing data is already dead, when DEF quality/doser quantity fail, or when efficiency stays low with both NOx sensors proven good (then SCR chemistry). System context: outlet NOx proves conversion; inlet NOx (SPN 3216) drives dosing. A lying outlet sensor is how fleets condemn good SCR bricks and walk into inducement on false efficiency.