What This FMI Means
FMI 8 indicates the signal's frequency, pulse width, or time relationship is outside the expected window for that circuit. This is a waveform-quality fault: the signal is present on the circuit, but its timing characteristics are wrong. It differs from FMI 2 (erratic signal) in being more specific — FMI 8 indicates the waveform pattern itself is incorrect, not just inconsistent.
FMI 8 applies primarily to frequency-based and pulse-width-modulated sensors: wheel speed sensors (which output a pulse frequency proportional to wheel speed), crankshaft and camshaft position sensors (which output a specific tooth-pattern waveform), and transmission speed sensors. The module decodes the pulse train and flags FMI 8 when the pattern does not match what it expects.
How It Appears With SPN Codes
Common SPN/FMI 8 combinations include ABS wheel speed sensor SPNs (SPN 904–907 for steer axle positions, higher-numbered SPNs for drive and trailer positions), crankshaft speed/position SPNs (SPN 636 on many engines), and camshaft position SPNs (SPN 637). On ABS controllers, wheel speed sensor FMI 8 is often caused by a damaged tone ring.
Crankshaft position FMI 8 combined with camshaft position FMI 8 may indicate an engine timing relationship problem — the cam-to-crank timing pattern is outside the expected window. This can follow a timing chain wear event, incorrect belt/chain installation, or a sensor air gap issue on one of the two sensors.
Common Causes to Investigate
Damaged reluctor rings (tone wheels) are the leading cause of FMI 8 on wheel speed sensors. Even one chipped, missing, or debris-packed tooth changes the pulse pattern enough to trigger FMI 8. Inspect by rotating the wheel and observing the reluctor ring face — damaged areas are usually visible with a flashlight without removing the sensor.
Incorrect sensor air gap is the second most common cause. The sensor must be within the manufacturer-specified distance from the reluctor ring. A gap that is too large produces a weak pulse; too small risks contact. Check gap with a feeler gauge per the OEM specification. Also inspect for metallic debris (brake dust, magnetic particles) bridging the gap between sensor tip and ring face.
What Drivers Should Record
Identify which axle or wheel position the fault code addresses — the SPN or fault description typically specifies the corner or position. Record whether ABS, traction control, or stability control warning lamps illuminated alongside the code.
For FMI 8 on engine sensors (crankshaft, camshaft), note whether the engine experienced a start concern, stall, rough-running, or power limitation when the code set. These events confirm whether the signal issue affected actual engine control or was detected by a background monitor during normal operation.
Related Pages
Related Fault Code Pages
- Bendix EC-60 UDS 9 / SPN 793 FMI 8
- Bendix EC-60 UDS 73 / SPN 789 FMI 8
- Bendix EC-60 UDS 105 / SPN 792 FMI 8
- Bendix EC-60 UDS 137 / SPN 790 FMI 8
- Bendix EC-60 UDS 169 / SPN 791 FMI 8
- Bendix EC-60 UDS 201 / SPN 794 FMI 8
- WABCO RSSplus SID 3 FMI 8
- WABCO RSSplus SID 4 FMI 8
- WABCO RSSplus SID 5 FMI 8
- WABCO RSSplus SID 6 FMI 8
- SPN 191 FMI 8
- SPN 191 FMI 8
- SPN 191 FMI 8
- SPN 191 FMI 8
- SPN 161 FMI 8
Sources
- SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - NHTSA Manufacturer Communications Search National Highway Traffic Safety Administration · government · accessed 2026-05-05 · confidence high
Source: National Highway Traffic Safety Administration, NHTSA Manufacturer Communications Search. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
Which types of sensors most commonly produce FMI 8 (abnormal signal timing) faults?
FMI 8 is most common on frequency-based and pulse-width-modulated sensors — wheel speed sensors (which output a frequency proportional to wheel speed), crankshaft and camshaft position sensors (which output a specific pulse pattern), and transmission speed sensors. Variable reluctance and Hall-effect sensors both output frequency-based signals. A damaged tone ring (reluctor wheel) is one of the most common causes.
Can a damaged reluctor ring (tone wheel) cause FMI 8 on a wheel speed sensor?
Yes — this is a common cause. A chipped, cracked, or debris-filled tone ring produces a distorted waveform. The ABS module samples the signal frequency and compares it to the expected pattern; an irregular pattern from a damaged ring produces FMI 8 rather than the voltage fault codes associated with an open or shorted circuit. Inspecting the tone ring is part of the diagnosis for wheel speed sensor FMI 8 codes.
How is FMI 8 different from FMI 2 (data erratic) for a speed or frequency sensor?
FMI 2 indicates the signal is inconsistent in a general way — the value changes unpredictably or is intermittently absent. FMI 8 is more specific: it indicates the waveform's frequency, pulse width, or timing relationship is outside the expected window for that signal type. An FMI 8 on a crankshaft position sensor often means the timing relationship between the crank and cam signals is wrong, which can indicate a timing issue or a sensor installation problem.