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SPN 1485 — ECM Main Relay

Safe to driveTo the nearest safe stop

Updated: 2026-11-14

J1939 code SPN 1485. Below — how the fault is worded at the factory by Isuzu, KAMAZ, MAZ, SITRAK and YaMZ, and what the second half of the code (FMI) means in the standard.

How to check it yourself

The main relay of the engine control unit. The unit switches the relay itself; through it the unit and some actuators are powered, and after the ignition is switched off the unit holds it on for a while to save data.

What it should read

Relay output voltage, ignition onas at the battery

Check order

  1. Find the relay from the diagram; listen for the click when the ignition is switched on.
  2. Swap the relay with an identical neighbouring one.
  3. The relay socket contacts: burnt, oxidised.
  4. The control wire of the relay from the unit.
  5. If the relay drops out straight after the ignition is switched off — the unit has no time to save data, and that is the source of the code.

Common causes — from most to least frequent

  1. The relay itself.
  2. The relay socket.
  3. Ground switch switched off right after the key.
  4. Control wire.

IsuzuKAMAZMAZSITRAKYaMZ

Isuzu

FMI 2Main relay system fault (Not enter) (P1625)

KAMAZ engine (J1939)

FMI 3Main relay 2
FMI 4Main relay 2

MAZ

FMI 3Main relay 2 circuit fault - Short to battery positive. Blink code: 1-3-2. Repair: Check the harnesses.
FMI 4Main relay 2 circuit fault - Short to ground. Blink code: 1-3-2. Repair: Check the harnesses.

SITRAK

FMI 3Main relay short circuit to power supply
FMI 5Main relay open circuit

YaMZ

FMI 3Main relay 2 circuit fault (Short to battery positive). Blink code: 1-3-2. Repair: Check the harnesses.
FMI 4Main relay 2 circuit fault (Short to ground). Blink code: 1-3-2. Repair: Check the harnesses.

What FMI means in J1939

SPN 1485 FMI 21485/2 · 1485.2data erratic or intermittent
SPN 1485 FMI 31485/3 · 1485.3voltage above normal or shorted high
SPN 1485 FMI 41485/4 · 1485.4voltage below normal or shorted to ground
SPN 1485 FMI 51485/5 · 1485.5current below normal or open circuit

Can I keep driving

Safe to drive. Below — how much time you have and what happens if you keep going.

What happens if you keep driving

Time you have: To the nearest safe stop

The electrical system is not holding voltage. While the engine runs the alternator carries it; shut it off and you may not restart.

Next the lights go, then modules start dropping out and false codes appear all over the truck. Get to somewhere with help without shutting the engine down.

What to check on the spot

Check the battery terminals and earth strap — corrosion and loose clamps throw false codes all over the truck. Running voltage should be 27–29 V.

Nearby: power supply

Related codes

Makes with this code

KAMAZMAZYaMZSITRAKIsuzu

Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.