B0120 indicates a fault with the fluid reservoir level sensor circuit E, meaning the vehicle's control module has detected an abnormal signal from the sensor that monitors the level of a specific fluid (commonly brake fluid) in its reservoir. The code is set when the sensor's output is outside the expected range or when the circuit is open/shorted.
The sensor itself may have internal electronic failure, causing an out‑of‑range signal.
Wires to the sensor can suffer chafing, corrosion, or loose pins, leading to an open or short circuit.
The sensor plug or the body control module (BCM) connector may not be fully seated or may have contaminated contacts.
Debris or water in the reservoir can coat the sensor element, altering its readings.
Insulation damage can cause the sensor line to contact voltage or chassis ground, producing an invalid signal.
Although uncommon, a failure in the module that reads the sensor can generate this code; this is considered only after sensor and wiring are verified good.
The definition is universal; the diagnostics are engine-specific. Open your vehicle for the codes documented for it:
No. The PCM/ECM is rarely the cause of B0120. Most cases are due to the fluid level sensor itself, its wiring, or connectors. Replacing the control module should only be considered after those components have been tested and found good.
Low fluid level alone usually sets a different warning (e.g., brake fluid low light). B0120 specifically indicates a sensor circuit fault, though extremely low fluid can sometimes affect sensor readings and contribute to the fault.
While the vehicle may still be drivable, the code indicates a potential loss of accurate fluid‑level monitoring, which could prevent early detection of a leak. It is advisable to diagnose and repair the issue promptly to maintain safety systems that rely on fluid level information.