NOx SUBSYSTEMe.g. SPN 3216 FMI 3
NOx Sensor Diagnostic Guide
NOx sensor diagnostics — upstream/downstream sensor plausibility, heater circuits, and cross-contamination detection.
Common Symptoms
- 1NOx plausibility fault code active
- 2NOx sensor voltage out of range
- 3Upstream/downstream NOx ratio abnormal
- 4NOx sensor heater circuit fault
- 5SCR efficiency calculated incorrectly
- 6Intermittent NOx fault on cold start
Root Causes (Ranked by Frequency)
- NOx sensor internal failure
- Sensor heater circuit open or short
- Connector corrosion or pushed-back pins
- Exhaust leak upstream of sensor
- DEF contamination affecting sensor reading
- Sensor installed in wrong position
Diagnostic Process — Step Preview
Structured diagnostic sequence — evidence-gated at each step
4 steps locked behind Technician Mode- 1INTAKE → sensor voltage baseline
- 2SENSOR_VALIDATION → heater circuit resistance
- 3PLAUSIBILITY_CHECKS → upstream vs downstream ratio
- 4ACTUATOR_TESTS → sensor heater command
- 5VERIFICATION → voltage within spec post-repair
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DOWNSTREAM EFFECTfix upstream first
DEF dosing failure → SCR efficiency loss → NOx plausibility
DEF dosing malfunction reduces SCR efficiency, causing NOx sensor plausibility codes that look like sensor failure.
DEF dosing malfunction → insufficient DEF injection
Insufficient DEF → SCR catalyst not reducing NOx
High NOx downstream → NOx sensor plausibility fault
Plausibility fault looks like sensor failure — check dosing first
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