SPN 3216: Aftertreatment 1 Intake NOx Sensor
What SPN 3216 Is
SPN 3216 is Aftertreatment 1 Intake NOx — engine-out / pre-SCR NOx. The ECM compares this to outlet NOx (3226) to compute conversion. It is not the efficiency code itself; that is SPN 4364.
Physically it sits in the exhaust before the SCR, often after the DPF on one-box systems. Same family of problems as outlet NOx: heater, CAN, drift, exhaust leaks upstream of this sensor (turbo-to-ATS) that pull in air and crash the reading.
Common FMI Combinations
| FMI | Meaning | Start here |
|---|---|---|
| 2 | Erratic | Exhaust leak before the sensor, connector |
| 3 / 4 / 5 | Circuit | Power, ground, CAN wiring |
| 9 | Abnormal update rate | Sensor off the private datalink |
| 20 / 21 | Drifted high / low | Rationality vs expected engine-out NOx |
Diagnostic Order
- Fix circuit and update-rate FMIs before you chase efficiency. A silent inlet sensor makes 4364 worthless.
- Smoke or listen for exhaust leaks from turbo outlet to the inlet NOx bung.
- At steady cruise, engine-out NOx should move with load. A flatline is a dead or stuck sensor.
- Do not replace both NOx sensors as a pair by default. Prove which one is lying.
We run this on a Nexiq USB-Link 3 with INSITE or DDDL. If you have the adapter on the truck, we connect. If you do not, we have the diagnostic laptop and adapter on our side.
Frequently Asked Questions
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