SPN 637 FMI 8 — Crankshaft position sensor abnormal signal

SPN 637 identifies the parameter the control module is monitoring — crankshaft position sensor abnormal signal. FMI 8 is the failure mode the module logged against it. Both numbers matter: the same SPN with a different FMI is a different repair.

Hard start, stall, no-start.

• Failed crank sensor
• Metal debris on the sensor tip
• Damaged tone wheel

• Scope the crank signal while cranking
• Pull the sensor and inspect the tip for debris
• Verify the tone wheel condition

Work the list in order and record the readings. Confirm the fault with live data before any part is ordered — a code names a circuit, never a failed component.

Short distance only

• Replacing the sensor named in the SPN before checking its wiring, connector and ground.
• Clearing the code and handing the truck back without reproducing the operating condition that set it.
• Reading one code out of a set instead of finding which fault occurred first.

Get a shop involved when the fault comes with a red lamp or a shutdown timer, when the checks above all pass and the code returns, or when the repair needs an aftertreatment service tool, a forced regen, or engine internals. Nothing on this page is a substitute for OEM service literature or a qualified technician on the truck.

Enter this code in Truck Commander Pro AI diagnostic support with the engine and the symptoms and you get the test sequence in order, per-truck history so repeat faults are visible, and a printable work order. Aftertreatment codes are covered in depth in the Aftertreatment Mastermind training. It is all in Truck Commander at $14.99 a month.