Use this library when
You have an SPN/FMI code, a dash warning, or a low-power complaint and need clearer context before calling a shop.
TruckHelpNow library
Truck fault code help for SPN/FMI diagnostics
These pages are built for real search intent around heavy-truck fault codes. Each entry explains the subsystem involved, why the code matters, what symptoms often show up, and what information helps before the truck is escalated to a technician.
What the pages include
Severity, driveability notes, likely subsystem impact, low-risk checks, and related procedures when available.
What they do not replace
OEM diagnostics, multimeter work, brake-system verification, or safety-critical inspection by a qualified technician.
Index
Browse current fault-code pages
Medium urgencyMay trigger reduced-power derate
SPN 4364 FMI 18
DEF quality or level – abnormal
Aftertreatment DEF (diesel exhaust fluid) quality or level fault. May trigger derate or warning.
Aftertreatment faults often affect regen behavior, DEF quality messages, and engine derate events.
View code detailsMedium urgencyMay trigger reduced-power derate
SPN 3710 FMI 2
DPF differential pressure – erratic or intermittent
DPF (diesel particulate filter) pressure sensor or circuit fault; can affect regen and derate.
Aftertreatment faults often affect regen behavior, DEF quality messages, and engine derate events.
View code detailsHigh urgencyCan create a no-start condition
SPN 91 FMI 3
Engine speed – voltage high
Engine speed sensor signal out of range high. May cause no-start or erratic idle.
Engine and sensor faults can show up as low power, rough running, hard starts, or no-start conditions.
View code detailsMedium urgencyMay require limp-home operation only
SPN 3226 FMI 4
Transmission output speed – voltage low
Output speed sensor fault; can affect shift quality and cruise.
Transmission faults can affect shift quality, speed sensing, cruise behavior, and limp-home operation.
View code detailsHigh urgencyCan be drivable, but still needs diagnosis
SPN 1214 FMI 5
Parking brake – current low
Parking brake circuit or switch fault. Safety-related; verify brake state before driving.
Brake and parking-brake warnings should be handled carefully because they can affect safe operation.
View code detailsMedium urgencyCan be drivable, but still needs diagnosis
SPN 97 FMI 11
Vehicle electrical power – root cause unknown
Vehicle power or charging fault; battery/alternator or load related.
Electrical and charging faults can create intermittent warnings, sensor drift, and multiple related alerts.
View code detailsMedium urgencyMay trigger reduced-power derate
SPN 96 FMI 3
Intake manifold pressure – voltage high
Boost pressure sensor or circuit fault; may limit power or set derate.
Engine and sensor faults can show up as low power, rough running, hard starts, or no-start conditions.
View code detailsHigh urgencyStop safely before continuing
SPN 51 FMI 4
Air compressor governor – voltage low
Air system pressure control fault; can affect brake reservoir and auxiliary air.
Electrical and charging faults can create intermittent warnings, sensor drift, and multiple related alerts.
View code details