B0121 means the body control module has detected an implausible, out-of-range, or failed signal from fluid reservoir level sensor "F." This sensor reports the level of a specific fluid reservoir, and the code sets when its voltage or resistance is outside the expected range. In most cases the problem lies with the sensor, its wiring, or the reservoir condition rather than a control module failure.
The sensing element can wear, stick, or produce an out-of-range resistance/voltage, causing the module to store the code.
Opens, shorts to ground or power, corrosion, or pin damage in the harness can mimic a faulty sensor.
Sludge, additive film, or the wrong fluid can alter the sensor reading and trigger a range/performance fault.
A cracked reservoir, displaced float, or fluid level outside the sensor's calibrated range can produce an implausible signal.
A faulty BCM or software calibration is possible only after the sensor, wiring, and reservoir have been thoroughly ruled out.
The definition is universal; the diagnostics are engine-specific. Open your vehicle for the codes documented for it:
Usually no. Most B0121 faults are caused by the fluid level sensor, its wiring, or the reservoir condition. Replacing the BCM is rarely the correct first repair.
Yes, in most cases it is safe to drive, provided the actual fluid level is correct and no related system is impaired. Repair it to restore accurate fluid monitoring.
It depends on the manufacturer; it may be the windshield washer, brake fluid, coolant, or another reservoir. Consult the service information for the specific vehicle.
No. If the underlying sensor, wiring, or reservoir problem remains, the code will usually return after a few key cycles.