Blog / Electrical & sensors
How to read J1939 codes (SPN and FMI) yourself
Most modern diesel engines in heavy equipment use a standard called J1939 for fault codes. Each code has two numbers: SPN and FMI.
What is SPN?
SPN is the number of the component or reading that has the problem. For example: 110 is engine coolant temperature, 100 is engine oil pressure, 97 is water in fuel.
What is FMI?
FMI describes the type of problem with that reading. The most common values:
- 0: reading dangerously above normal.
- 1: reading dangerously below normal.
- 2: reading erratic or invalid.
- 3: voltage above normal (usually a wiring issue).
- 4: voltage below normal (usually a wiring issue).
- 15 to 18: reading out of range at a less severe level.
Example
SPN 110 FMI 0 means engine coolant temperature is dangerously high. SPN 110 FMI 4 usually means a sensor wiring problem, not real overheating.
⚠️ Some brands like CAT also have their own codes (such as E360) alongside J1939 codes. Type the code into our search and we'll explain it.
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