SPN 701 — Auxiliary Input/Output #01
Safe to driveDrivable, but watch it yourself
Updated: 2026-11-14
J1939 code SPN 701. Below — how the fault is worded at the factory by Mercedes-Benz, Meritor (ex-WABCO) and Renault Trucks, and what the second half of the code (FMI) means in the standard.
How to check it yourself
Additional control unit output No. 1: it switches the differential lock. The number in the factory table is the unit's internal number, which does not match across makes. So the search is by the circuit, not by the number: what is connected to this output — check against the manual.
What it should read
| Output voltage with the load off | about zero |
| Load resistance (valve or relay) | per the manufacturer's manual |
Check order
- FMI 3, 4 (3 — short to plus, 4 — to ground): with the ignition off, unplug the load connector and switch the ignition on. If the code stays — a short in the wiring to the load. If the code goes — the load is faulty.
- FMI 5 (open circuit): check the wire from the output to the load, and measure the load resistance; compare with the manual.
- Factory table entries: Meritor — differential lock; Mercedes — digital output 3/07. Before replacing anything, check which load is actually fitted to this output.
Common causes — from most to least frequent
- Breakdown or overheating of the valve or load relay.
- Wire in the harness chafed at the bends.
- Oxidised load connector.
- Short to plus in the trailer harness or in the cab wiring.
Mercedes-BenzMeritor (ex-WABCO)Renault Trucks
Mercedes-Benz
| FMI 3 | Digital Output 3/07 Circuit shorted to Ubat |
| FMI 4 | Digital Output 3/07 Circuit shorted to GND |
| FMI 5 | Digital Output 3/07 Open Circuit (broken wire, terminal floating) |
Meritor (ex-WABCO)
| FMI 3 | Failure detection 'ShortUb' at Auxiliary Output (Diff Lock) |
| FMI 4 | Failure detection 'ShortGnd' at Auxiliary Output (Diff Lock) |
| FMI 5 | Failure detection 'Interruption' at Auxiliary Output (Diff Lock) |
| FMI 13 | Failure detection 'OverEquipped' at Auxiliary Output (Diff Lock) |
Renault Trucks
| FMI 3 | Reverse light driver, double-H shift pattern, short to positive. Repair: Check the continuity and insulation of the wiring with respect to +24 V (SL1/18). |
| FMI 4 | Reverse light driver, double-H shift pattern, open circuit or short to ground. Repair: Check the continuity and insulation of the wiring with respect to ground (SL1/18). |
What FMI means in J1939
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
This is an electrical fault: an open, a short or a bad contact in the sensor circuit. The value it measures is most likely fine.
You can drive. But until the circuit is repaired this system runs blind and cannot warn you about a real failure.
Nearby: other
- SPN 564Inter-Axle Differential Lock Status
- SPN 609Controller #2
- SPN 611System Diagnostic Code #1
- SPN 624Yellow Warning Lamp
- SPN 630Calibration Memory
- SPN 705Auxiliary Input/Output #05
- SPN 706Auxiliary Input/Output #06
- SPN 711Auxiliary Input/Output #11
- SPN 723Engine speed (auxiliary sensor)
- SPN 732Gas Metering Valve
Related codes
- SPN 746Differential Lock Solenoid Valve #1
- SPN 806ASR Valve, Differential 1
- SPN 558Accelerator Pedal Idle Limit Switch #1
- SPN 533Maximum Momentary Override Speed, Point 7
- SPN 527Cruise Control States
- SPN 920Audible Alarm
- SPN 925Auxiliary Output #2
- SPN 970Engine Auxiliary Shutdown Switch
Makes with this code
Mercedes-BenzRenault TrucksMeritor (ex-WABCO)
Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.