Do not driveTo the nearest safe stop
SPN 121 is a manufacturer number outside the standard J1939 table: different makes can put different things under it. Below — how it is worded by Renault Trucks and ZF AS-Tronic, and what the second half of the code (FMI) means.
| FMI 0 | Mechanical fault of the large disengagement solenoid valve. Repair: Run the diagnostic check of the clutch actuator. Check the clutch. Switch the ignition off, switch it on again and repeat the test. If the fault persists, replace the control unit. |
ZF AS-Tronic uses its own fault numbering: the number in the table below is an internal ZF code, not a J1939 SPN. It matches the title of this page by digit only and means something different.
| FMI 0 | Mechanical fault in valve Y16. |
| SPN 121 FMI 0121/0 · 121.0 | data above normal range (most severe level) |
Do not drive. Stop as soon as it’s safe to do so — don’t keep going.
The code is flagged critical: the manufacturer considers it unsafe to keep driving without checking.
This code alone cannot say what will fail next, so stop at the first safe opportunity and look at it on the spot.
That said, the manufacturer flags this code as critical — if in doubt, it is safer to stop and check on the spot.
Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.