C109A

Code

Generic C — Chassis

Brake Pressure Sensor Circuit Range/Performance

Brand: Generic

Type: C — Chassis

Views: UK: 6 EN: 5 RU: 8

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Page language: EN

Causes

Damaged or corroded sensor connector or wiring (open, short to voltage, short to ground, high resistance)
Failed brake pressure (hydraulic) sensor
Loss of sensor reference supply (usually 5 V) or poor ground at module
Contaminated or seized hydraulic component affecting pressure sensing
Faulty ABS/ESC control module or internal amplifier
Incorrect sensor calibration or software fault

Symptoms

ABS and/or traction control (ESC/TCS) warning light illuminated
ABS may be disabled or inactive; possible traction control faults
Illumination may be steady or intermittent depending on fault
Possible ABS pump activation or unusual brake feel during self-tests
Related drivability messages or reduced stability features
Stored freeze-frame and related readouts in scan tool

What to check

Read codes and freeze frame data with a capable scan tool; record occurrence conditions
Visual inspection of sensor, connector, and harness for damage, corrosion, moisture
Check brake fluid level and basic hydraulic condition
Backprobe sensor connector to check reference voltage, signal, and ground
Wiggle harness and connectors while monitoring live data for intermittent changes
Measure continuity and resistance between sensor and control module pins with ignition off

Signal parameters

Typical sensor type: analog ratiometric pressure sensor (varies by vehicle)
Common reference supply: ~5.0 V (check vehicle spec)
Expected signal range (typical): ~0.5–4.5 V across pressure range; near 0.5 V at low pressure, up to ~4.5 V at high pressure
Some systems may use frequency or digital sensors — consult OEM spec
Signal should be stable with slow change during steady pressure and should vary smoothly during brake application

Diagnostic algorithm

Connect a scan tool and read C109A plus any other ABS/traction codes and freeze-frame data. Note vehicle state when fault set (ignition on, key off, during braking, during regen, etc.).
Perform a visual inspection of the brake pressure sensor, connector, and wiring harness for corrosion, damage, pin push-out, or water ingress.
With ignition ON (engine off unless specified), backprobe the sensor connector: verify reference voltage (≈5 V), measure signal voltage at rest and while an assistant applies the brake pedal, and confirm a good ground. Compare to OEM values.
If reference or ground missing, trace and repair power/ground circuit to ABS module. Check fuses and relays that supply the module/sensor if applicable.
Check continuity and resistance between sensor pins and ABS module pins with ignition OFF. Repair any opens or high-resistance joints.
Wiggle test the harness and connectors while monitoring live data to identify intermittent faults. Repair or replace harness sections as needed.
If wiring and supply are good but signal out of range or non-linear, swap in a known-good sensor or substitute per service manual to verify sensor health.
If replaced sensor shows same fault and wiring verified, inspect hydraulic components (pressure modulator, valve block) for internal faults that could affect sensor behavior.
If all hardware checks good, consult OEM wiring diagrams and consider module bench test or replacement; check for available software/calibration updates.
After repair, clear codes and perform road or brake testing to confirm fault does not return. Re-scan for related or pending codes.

Likely causes

Wiring/connectors damaged or corroded at sensor or harness
Failed brake pressure sensor
Loss of 5 V reference or ground at sensor
Contaminated hydraulic circuit or faulty brake modulator
Module internal fault (less common)

Fault status

⚠️ Status

Brake Pressure Sensor Circuit Range/Performance — sensor output is outside expected range or behaves implausibly; may disable ABS/ESC and trigger warning lights.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.5 hours

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