Home › Fault codes › Aftertreatment & Emissions
Aftertreatment & Emissions fault codes
The codes that park a truck without anything mechanical breaking: DPF differential pressure, DEF quality and dosing, SCR NOx conversion, EGR flow, valve and cooler faults, and the regen logic tying them together. EGR lives here rather than with the engine — it is an emissions device plumbed into the air path, and it fails for emissions reasons. Most of these stage a soft derate first and a 5 mph derate a few hundred miles later, so the clock starts the moment the lamp lights.
All 100 aftertreatment & emissions SPN/FMI and OBD codes in the knowledge base are listed below — each with what it means, how serious it is, the most-likely causes, and what to check first.
Derated or broken down right now?
Aftertreatment & Emissions codes (100)
SPN 3251 · FMI 2Serious
DPF Differential Pressure — Data Erratic
SPN 3361 · FMI 7SeriousDEF Dosing Unit — Not Responding Properly
SPN 5246 · FMI 0CriticalSCR Operator Inducement — Severe Derate Active
SPN 1761 · FMI 1CautionDEF Tank Level — Low
SPN 4364 · FMI 18SeriousSCR Conversion Efficiency — Below Threshold
SPN 4094 · FMI 31SeriousDEF Quality — Out of Spec
SPN 3719 · FMI 16CautionDPF Soot Load — High (Regen Needed)
SPN 3216 · FMI 20CautionInlet (Upstream) NOx Sensor — Data Drifted
SPN 411 · FMI 3CautionEGR Differential Pressure — Voltage High
SPN 412 · FMI 0SeriousEGR Temperature — Too High
SPN 5394 · FMI 7SeriousDEF Dosing Valve — Not Responding
SPN 411 · FMI 0SeriousEGR diff pressure high
SPN 411 · FMI 2SeriousEGR diff pressure — erratic
SPN 1761 · FMI 18SeriousDEF tank level low
SPN 2659 · FMI 0SeriousEGR 1 Differential Pressure - Data Valid But Above Normal Operating Range (EGR flow/differential pressure too high)
SPN 2791 · FMI 7SeriousEGR valve — not responding
SPN 3031 · FMI 3SeriousDEF tank temp sensor — voltage high
SPN 3216 · FMI 2SeriousSCR inlet NOx sensor — erratic
SPN 3216 · FMI 9SeriousUpstream NOx sensor — abnormal update rate
SPN 3216 · FMI 12SeriousUpstream NOx sensor — internal failure reported
SPN 3224 · FMI 18CautionSCR NOx conversion efficiency below threshold
SPN 3226 · FMI 2SeriousDownstream NOx sensor — erratic
SPN 3226 · FMI 9SeriousDownstream NOx sensor — abnormal update rate
SPN 3226 · FMI 12SeriousDownstream NOx sensor — internal failure
SPN 3241 · FMI 3SeriousDPF inlet temp sensor — voltage high
SPN 3242 · FMI 2SeriousDPF outlet temp erratic
SPN 3246 · FMI 3SeriousDPF outlet temp sensor — voltage high
SPN 3251 · FMI 0CriticalAftertreatment 1 Diesel Particulate Filter Differential Pressure - Data Valid But Above Normal Operating Range, Most Severe Level (DPF severely restricted / plugged)
SPN 3251 · FMI 3SeriousDPF differential pressure — voltage high
SPN 3251 · FMI 4SeriousDPF differential pressure — voltage low
SPN 3361 · FMI 5SeriousDEF dosing valve solenoid — open circuit
SPN 3361 · FMI 6SeriousDEF dosing valve solenoid — short to ground
SPN 3361 · FMI 11SeriousDEF dosing unit — root cause unknown
SPN 3361 · FMI 31SeriousDEF dosing unit — condition exists
SPN 3363 · FMI 5SeriousDEF tank heater — open circuit
SPN 3363 · FMI 6SeriousDEF tank heater — short to ground
SPN 3364 · FMI 18CautionDEF concentration below target
SPN 3700 · FMI 31InformationalDPF active regeneration in progress
SPN 3703 · FMI 31InformationalDPF regeneration inhibited
SPN 3711 · FMI 31InformationalForced regeneration requested/in progress
SPN 3712 · FMI 15SeriousSCR operator inducement — stage 1 (warning only)
SPN 3712 · FMI 16CautionSCR inducement — stage 2 (25% torque reduction)
SPN 3712 · FMI 17CautionSCR inducement — stage 3 (severe torque derate)
SPN 3712 · FMI 18CautionSCR inducement — stage 3 (severe derate)
SPN 3712 · FMI 31CautionSCR inducement — condition exists
SPN 3719 · FMI 0CautionDPF soot load very high — service required
SPN 3719 · FMI 15SeriousDPF soot load high — warning
SPN 3720 · FMI 0SeriousDPF ash load very high — service required
SPN 4334 · FMI 3SeriousDEF pump pressure sensor — voltage high
SPN 4334 · FMI 4SeriousDEF pump pressure sensor — voltage low
SPN 4334 · FMI 18CautionDEF pump pressure below target
SPN 4335 · FMI 11SeriousDEF pump — root cause unknown
SPN 4364 · FMI 1CriticalAftertreatment 1 Diesel Exhaust Fluid Quality - Data Valid But Below Normal Operating Range, Most Severe Level (DEF quality/concentration incorrect)
SPN 4374 · FMI 5SeriousDEF pump relay — open
SPN 4376 · FMI 3SeriousDEF dosing unit temperature — voltage high
SPN 4376 · FMI 4SeriousDEF dosing unit temperature — voltage low
SPN 4383 · FMI 5SeriousDEF line heater 1 — open
SPN 4384 · FMI 5SeriousDEF line heater 2 — open
SPN 4385 · FMI 5SeriousDEF line heater 3 — open
SPN 4732 · FMI 2SeriousDEF quality sensor — erratic readings
SPN 4765 · FMI 3SeriousDOC inlet temp sensor — voltage high
SPN 4766 · FMI 3SeriousDOC outlet temp voltage high
SPN 5246 · FMI 16CautionSCR NOx limit — moderate derate
SPN 5298 · FMI 7SeriousDEF dosing start permission — not responding
SPN 5394 · FMI 5SeriousDEF dosing valve solenoid — open circuit
SPN 5394 · FMI 6SeriousDEF dosing valve solenoid — short to ground
SPN 5742 · FMI 2SeriousDEF quality erratic
SPN 5967 · FMI 15SeriousSCR ammonia slip high
P0401CautionEGR Flow Insufficient Detected
P0420SeriousCatalyst Efficiency Below Threshold
P0430SeriousCatalyst System Efficiency Below Threshold Bank 2
P0545CautionExhaust Gas Temperature Sensor Circuit Low Bank 1
P0549CautionExhaust Gas Temperature Sensor Circuit High Bank 2
P2002SeriousDiesel Particulate Filter Efficiency Below Threshold
P2003SeriousDPF Ash Accumulation Restriction
P2033CautionEGT Sensor Circuit Range Bank 1
P2047SeriousReductant Injector Circuit Open
P2048SeriousReductant Injector Circuit Low
P204FCriticalReductant System Performance Bank 1
P2077CautionEGR Control Circuit Range
P20EACriticalReductant Quality Deviation
P20EESeriousSCR NOx Catalyst Efficiency Below Threshold — Bank 1
P2200SeriousNOx Sensor Circuit Bank 1
P2201SeriousNOx Sensor Circuit Range Bank 1
P2202SeriousNOx Sensor Circuit Low Bank 1
P2203SeriousNOx Sensor Circuit High Bank 1
P2413SeriousEGR System Performance
P2452CautionDPF Pressure Sensor A Circuit
P2453CautionDPF Differential Pressure Sensor Range
P2458CautionDPF Regeneration Duration
P2459CautionDiesel Particulate Filter Regeneration Frequency
P2460CriticalDPF Soot Accumulation Restriction
P2463SeriousDiesel Particulate Filter — Soot Accumulation
P249ACautionParticulate Filter Differential Pressure Too Low at Idle
P249DSeriousParticulate Filter Restriction - Forced Regen Required
P24A0SeriousReductant Pressure Too Low
P24A1CautionReductant Pressure Too High
P24B0CautionReductant Heater Control Circuit Low
P24B1CautionReductant Heater Control Circuit High
P24C0SeriousNOx Absorber Catalyst Efficiency Below Threshold
Other systems
Drivetrain & Transmission
61 codes
Engine58 codes
Brakes, ABS & Stability41 codes
Fuel System39 codes
Electrical, Charging & Network30 codes
Cooling, lubrication, HVAC and chassis/cab codes are listed on the fault code lookup, along with the search box that covers every code.
General diagnostic guidance for heavy-duty diesel equipment (Cummins, Detroit, PACCAR, Volvo, Mack and more). Not a substitute for a scan-tool diagnosis on your specific truck.