SPN 46 FMI 1 — Air system pressure critically low

SPN 46 identifies the parameter the control module is monitoring — air system pressure critically low. FMI 1 is the failure mode the module logged against it. Both numbers matter: the same SPN with a different FMI is a different repair.

Spring brakes will apply. Truck stops itself.

• Air leak in a line or chamber
• Failed compressor or governor
• Air dryer purge valve stuck open

• Listen for the leak with the engine off
• Check governor cut-in and cut-out
• Inspect the air dryer purge valve

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

Absolutely not. Below 60 PSI the spring brakes set on their own, wherever you happen to be. Park now.

• 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.