SPN 1692 FMI 18 — Turbocharger underboost

SPN 1692 identifies the parameter the control module is monitoring — turbocharger underboost. FMI 18 is the failure mode the module logged against it. Both numbers matter: the same SPN with a different FMI is a different repair.

Noticeable power loss, sluggish acceleration.

• Charge air cooler or boot leak
• Sticking VGT actuator
• Restricted air filter

• Pressure test the charge air system
• Command the VGT actuator through its sweep
• Check air filter restriction gauge

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

You can limp it home slowly if the run is short, but expect a derate. Do not haul heavy or climb grades on it.

• 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.