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Home / DTC / C05CA — Brake Master Cylinder Piston Position Sensor A Circuit Voltage High

C05CA — Brake Master Cylinder Piston Position Sensor A Circuit Voltage High

Detailed page for trouble code C05CA.

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C05CA

Generic C — Chassis

Brake Master Cylinder Piston Position Sensor A Circuit Voltage High

Brand: Generic
Type: C — Chassis
Views: UK: 15 EN: 20 RU: 15
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Page language: EN

Causes

  • Short to battery on the sensor signal or supply circuit
  • Open or poor ground for the sensor or ECU ground
  • Faulty master cylinder piston position sensor (internal short/failed electronics)
  • Corroded, loose or damaged connector or wiring harness
  • Water intrusion or contamination in connector
  • Fault in ABS/ECU input stage or reference supply

Symptoms

  • Brake warning lamp, ABS or traction control warning illuminated
  • Change in brake pedal feel or reduced brake assist
  • ABS or stability control may be disabled or degraded
  • Stored fault in ABS or brake system control module
  • Possible limp or reduced functionality of associated systems

What to check

  • Read freeze frame and stored data with a scan tool; note voltage and conditions when the fault set
  • Visually inspect master cylinder sensor connector, wiring and routing for damage, corrosion or pin push-out
  • Backprobe the sensor connector and record signal and reference voltages with ignition ON
  • Check continuity and resistance between sensor signal pin and ECU harness pin (with power off)
  • Verify proper ground at the sensor and at the ABS/control module ground points
  • Wiggle test wiring while watching live data for intermittent changes

Signal parameters

  • Typical reference supply: 5 V (sensor powered from ECU/reference circuit) — vehicle dependent
  • Normal sensor output range: approximately 0.5–4.5 V under operation (varies by design)
  • Voltage considered high: signal near supply voltage or >4.8–5.0 V, or reading near battery voltage (~12 V)
  • Expected idle/rest sensor reading: mid-range (vehicle/manufacturer specific) — consult OEM data

Diagnostic algorithm

  1. Connect a professional scan tool, retrieve freeze frame and live sensor data; note exact voltage when code set.
  2. Visually inspect sensor, connector and harness for obvious damage, signs of corrosion, or fluid contamination; repair as needed.
  3. With ignition ON (engine off), backprobe connector: measure reference (5 V), signal, and ground. Record values. A signal at or near supply or battery voltage indicates a high-voltage condition.
  4. If signal equals battery voltage, inspect for a short to ignition or battery feed on the signal or supply wire. Isolate circuit sections and test continuity to battery when harness is disconnected.
  5. Check continuity and resistance between sensor ground and chassis ground/ECU ground; fix any high-resistance ground.
  6. If wiring and grounds test good, disconnect sensor and measure signal wire at harness connector — if voltage returns to normal, suspect sensor internal fault.
  7. If possible, substitute a known-good sensor or temporarily simulate expected sensor resistance/voltage per OEM procedure to confirm ECU response.
  8. Inspect and test the ABS/brake control module reference output and input pin for damage. Replace module only after eliminating wiring and sensor faults.
  9. After repair, clear codes, perform functional tests and road test to verify the fault does not return.

Likely causes

  • Damaged insulation with conductor contacting constant +12 V
  • Corroded pin or connector causing intermittent high reading
  • Failed sensor electronics producing high output voltage
  • Faulty 5 V reference or pull‑up in the ABS/BCM module
  • Poor ground at sensor or ECU causing improper reference

Fault status

⚠️ Status
C05CA — Brake Master Cylinder Piston Position Sensor A Circuit Voltage High: sensor signal above expected range (possible short to battery, sensor failure, connector/wiring or ECU reference/ground issue).
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-2.5 hours

Similar codes

C05C9 C05C0 C05CB U0121 P0500
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