Error code · Trucks: SPN codes

SPN 46 — Pneumatic supply pressure

Don’t put off the repair

Open this code in the appDownload the app

A fault code doesn’t yet mean the unit or sensor itself is faulty: often it’s the plug and connector leading to it.

What you can do yourself

  1. This is the overall air pressure that supplies the air system: the brakes and other air consumers depend on it.
  2. Check the compressor, the pressure regulator and air leaks (listen for hissing).
  3. Most often the air escapes around the air dryer — through its gaskets and fittings.
  4. FMI 0, 15, 16 — pressure above normal: a faulty pressure regulator (governor) or air dryer — go to an air brake specialist.
  5. FMI 2, 3, 4, 5 — the pressure sensor, its connector or wiring — go to an auto electrician.
  6. Electrics (sensors, valves, wiring, control unit) — to an auto electrician. Leaks and mechanical faults — to an air brake specialist.

Stop and don’t drive on if:

  • The low air pressure light is on or the buzzer sounds
  • The air pressure gauges on the dashboard are below the normal range or dropping
  • The brakes have got worse, or the vehicle is braking by itself

Add the FMI — the second number of the code. It shows what exactly is wrong with the parameter.

Source: SAE J1939-71: J-1939 SA/SPN/FMI Descriptions (static.nhtsa.gov, MC-10141869)

The explanation comes from the error code reference in the AvtoElektrik PRO app.

The second number, FMI: what exactly is wrong

A truck code is two numbers: SPN (which part) and FMI (what is wrong with it). Tap your FMI and the app opens the meaning of that exact code.

With this FMI — stop: FMI 1, 17, 18

Low air in the brakes: stop in a safe place, switch on the hazard lights and don’t drive on — the brakes work on air. Don’t move off until the pressure builds up to normal and the low air pressure light goes out.

Need a mechanic’s help?

Couldn’t fix it yourself? Oleg, an auto electrician, does computer diagnostics and repairs: he comes out anywhere in Almaty or sees you at his workshop.

Truck computer diagnostics in AlmatyCall: +7 701 121 11 10

Want to figure it out yourself?

Oleg’s course “Car computer diagnostics with a scan tool”: how to hook up a scanner, read codes and live data, and use them to find the cause. The first lessons are free.

Scan tool diagnostics course →

Nearby codes

All codes in Trucks: SPN codes →

Who keeps this reference

The reference is kept by Oleg Dmitriev, an auto electrician from Almaty with 26 years of experience: computer diagnostics of cars and trucks, repair of control units (ECUs). Can’t find the cause? Message him.

Oleg’s auto electrician websiteWhatsApp +7 701 121 11 10