SPN 177 FMI 0 — Transmission oil temperature too high

SPN 177 identifies the parameter the control module is monitoring — transmission oil temperature too high. FMI 0 is the failure mode the module logged against it. Both numbers matter: the same SPN with a different FMI is a different repair.

Shift protection, risk of clutch damage.

• Heavy load on a long grade
• Low or degraded fluid
• Cooler blockage

• Pull over and idle to cool
• Check fluid level and smell
• Inspect the transmission cooler for blockage

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.

Park it — do not drive

Cool it down before moving. Continuing home hot cooks the clutches.

• 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 Blue Collar 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.