Safe to driveDrivable, but watch it yourself
J1939 code SPN 3871. Below — how the fault is worded at the factory by Mercedes-Benz, and what the second half of the code (FMI) means in the standard.
| FMI 5 | Transmission Speed Sensor Circuit Open (broken wire, terminal floating) |
| SPN 3871 FMI 53871/5 · 3871.5 | current below normal or open circuit |
Safe to drive. Below — how much time you have and what happens if you keep going.
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.
Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.