SPN 3216 FMI 2 — SCR inlet NOx sensor — erratic
Applies to: Detroit · Navistar · Paccar · Sae · Volvo · A26 with SCR · NOx sensors are wear items
SCR inlet NOx sensor — erratic.
Look up listed providers near you and give them this SPN/FMI when you call.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- NOx sensor aging (normal consumable wear ~300k miles)
- Heater circuit degraded
- Connector contamination
- NOx sensor aging
- Heater issue
- Wiring
- NOx aging
First moves (roadside)
- Wait 60-90s after warm start for heater cycle
- Check connector
- Compare inlet vs outlet NOx readings
- Allow heater warmup
- Check connectors
- Warmup
- Connector
Common misdiagnoses
- Replacing upstream before outlet — check which is drifting
FAQ
SCR inlet NOx sensor — erratic.
Serious — Power derate likely — plan to stop soon.
The most common cause is: NOx sensor aging (normal consumable wear ~300k miles). Other possibilities include Heater circuit degraded; Connector contamination; NOx sensor aging; Heater issue; Wiring; NOx aging.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.