FMI Codes 0–31: Failure Mode Identifiers
What an FMI Tells You
The Failure Mode Identifier is a number from 0 to 31 attached to an SPN. It does not name the part. It names the mode: the signal is too high, the circuit is open, the module stopped talking, or a protection condition is active.
Same SPN, different FMI, different job. SPN 3226 FMI 4 is an outlet NOx circuit (voltage low). SPN 3226 FMI 20 is a rationality / drift fault — you may have a sensor that talks but lies, or DEF crystallization in the mixer throwing the reading off.
FMI 0–31 Table
| FMI | SAE description | Shop translation |
|---|---|---|
| 0 | Data valid, above normal — most severe | Reading is real and way too high |
| 1 | Data valid, below normal — most severe | Reading is real and way too low |
| 2 | Erratic, intermittent, or incorrect | Connector, chafe, contamination |
| 3 | Voltage above normal / shorted high | Short to power |
| 4 | Voltage below normal / shorted low | Short to ground or open on a voltage circuit |
| 5 | Current below normal / open circuit | Open: unplugged, broken wire, failed coil |
| 6 | Current above normal / grounded circuit | Shorted actuator, excessive draw |
| 7 | Mechanical system not responding | Stuck, seized, out of adjustment |
| 8 | Abnormal frequency / pulse width / period | Speed or PWM sensor timing |
| 9 | Abnormal update rate | Message missing on the data link |
| 10 | Abnormal rate of change | Changed faster than physics allows |
| 11 | Root cause not known | Failure detected, mode unclear |
| 12 | Bad intelligent device or component | Smart sensor / module internal |
| 13 | Out of calibration | Needs programming, trim, or alignment |
| 14 | Special instructions | OEM procedure / TSB — do not guess |
| 15 | Above normal — least severe | Informational high |
| 16 | Above normal — moderately severe | Investigate |
| 17 | Below normal — least severe | Monitor |
| 18 | Below normal — moderately severe | Common on SCR efficiency |
| 19 | Received network data in error | Bad payload on CAN |
| 20 | Data drifted high | Rationality high (NOx sensors) |
| 21 | Data drifted low | Rationality low |
| 22–30 | Reserved / manufacturer | Look up the OEM overlay |
| 31 | Condition exists | Derate, inducement, or a latched state |
How to Use FMI on the Bench
Do not buy the SPN part until the FMI agrees. FMI 5 on a dosing valve is an open circuit — check the connector before the valve. FMI 7 on a VGT is mechanical — soot-bound vanes or a dead actuator, not a 5-volt reference. FMI 9 means the ECM is not receiving the message at the expected rate: terminating resistors, private aftertreatment datalink, or a sensor that has dropped off CAN.
Pair this table with the SPN/FMI hub and the individual SPN pages. If you need OEM software on the truck tonight, we run sessions with a Nexiq USB-Link 3 and a diagnostic laptop.
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