SCR SUBSYSTEMe.g. SPN 3031 FMI 5
SCR Diagnostic Guide
Selective Catalytic Reduction system diagnostics — NOx efficiency, DEF quality, dosing injector, and catalyst health.
Common Symptoms
- 1NOx efficiency below threshold (SPN 3031)
- 2DEF quality fault or contamination
- 3SCR catalyst derate active
- 4Dosing injector fault codes
- 5Excessive NOx output at tailpipe
- 6DEF consumption abnormally high or low
Root Causes (Ranked by Frequency)
- NOx sensor failure (upstream or downstream)
- DEF contamination or wrong fluid
- Dosing injector clogged or failed
- SCR catalyst degraded or poisoned
- DEF supply pump failure
- Frozen DEF lines in cold weather
Diagnostic Process — Step Preview
Structured diagnostic sequence — evidence-gated at each step
4 steps locked behind Technician Mode- 1INTAKE → NOx sensor voltage check
- 2SENSOR_VALIDATION → DEF quality sensor
- 3ACTUATOR_TESTS → dosing injector command
- 4FLOW_PRESSURE_CHECKS → DEF supply pressure
- 5VERIFICATION → NOx efficiency post-repair
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UPSTREAM CAUSEfix 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
UPSTREAM CAUSEparallel investigation
DEF quality low → SCR catalyst degradation
Contaminated or low-concentration DEF can poison the SCR catalyst over time.
Low DEF concentration or contamination
Improper chemical reaction in SCR catalyst
Catalyst poisoning / degradation over time
SCR efficiency drops → NOx emissions increase
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