C05BF

Code

Generic C — Chassis

Brake Booster Motor B Position Sensor Supply Circuit Range/Performance

Brand: Generic

Type: C — Chassis

Views: UK: 31 EN: 34 RU: 25

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Completed

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Page language: EN

Causes

Open or short in the sensor supply (reference voltage) or ground circuit
Poor or corroded connector or pin contact at the sensor or control module
Damaged wiring (chafing, crushed, water intrusion)
Faulty brake booster motor B position sensor
Intermittent connection or high-resistance circuit causing voltage drop
Control module power/ground or internal driver fault

Symptoms

Brake/ABS/ESC warning lamp illuminated
Reduced or lost brake assist (increased pedal effort) or degraded brake feel
Diagnostic trouble code stored (C05BF) and possibly related chassis codes
Inconsistent or erratic position reading for brake booster motor B in live data
Possible limp or reduced functionality of brake-brake-assist systems

What to check

Retrieve all stored codes and freeze frame/fault data with a capable scan tool
Check live data for motor B position sensor values while commanding/operating the booster
Visually inspect sensor connector and harness for corrosion, damage, or water ingress
Backprobe sensor connector with key ON (engine OFF) to measure reference, signal and ground voltages
Perform wiggle test on harness and connector while monitoring live data or voltage
Measure resistance/continuity of wiring between sensor connector and control module with connector disconnected

Signal parameters

Sensor reference voltage: typically 5.0 V ± 0.5 V (verify with vehicle-specific documentation)
Sensor ground: near 0 V, low resistance to chassis ground (should be < 1–2 Ω measured at harness if specified)
Sensor output signal: variable analog voltage across travel — commonly expected to sweep roughly 0.1–4.9 V or 0.5–4.5 V depending on design (consult model datasheet)
Typical sensor supply current: small milliamps (often < 20 mA) for Hall/ratiometric sensors
If PWM-type output is used, frequency often in the low hundreds of Hz — use scope to verify waveform

Diagnostic algorithm

Connect a scan tool, read and record DTCs and freeze frame data. Note any simultaneous codes and stored conditions.
With key ON (engine OFF), backprobe the sensor connector: verify reference voltage (≈5 V), signal and ground presence. If reference is missing, trace back to fuse/module power.
If reference is present but signal is out of range or absent, check continuity between sensor ground and module ground; repair high-resistance or open ground.
Wiggle the harness and connectors while observing live PID or voltage to find intermittent connections.
If voltages look plausible but signal does not change with booster movement, command the actuator (if possible) or move the booster linkage and observe signal sweep. If no sweep, suspect sensor failure.
Measure resistance/continuity from the sensor connector to the control module connector pins with harness disconnected. Look for opens, shorts to battery or ground, or low insulation resistance to chassis.
Use an oscilloscope to inspect the signal for noise, dropouts, or PWM characteristics if present. Compare waveform to expected pattern.
Repair any damaged wiring, clean/replace corroded connectors, or replace the sensor if it fails static and dynamic tests.
If wiring and sensor test good, test or replace the control module only after verifying all external circuits are correct.
Clear codes and perform a road/test rig verification and final scan to ensure the fault does not return.

Likely causes

Loss of 5V (or module reference) to the sensor due to harness damage or blown fuse
Sensor output stuck at rail (near 0V or near supply) from internal sensor failure
Short to battery voltage or to ground on the signal or supply wire
Corroded sensor connector allowing intermittent supply
Faulty module pin or module internal supply circuit

Fault status

⚠️ Status

Brake Booster Motor B position sensor supply circuit out of range/poor performance — check sensor supply (reference), ground, signal, wiring and connectors; may cause brake/ABS warnings and reduced assist.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours