Code
C1691
Generic
C — Chassis
Brake hydraulic pressure sensor circuit malfunction
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AI status
Completed
Completed
100%
Causes
- Open or shorted wiring in brake pressure sensor circuit
- Corroded, loose or damaged sensor connector
- Failed brake hydraulic pressure sensor
- Blown fuse or lost sensor reference/ground
- Faulty ABS/traction control (brake) control module or internal driver
- Intermittent contact from chafed harness or water intrusion
Symptoms
- ABS/ESC warning lamp illuminated
- Brake system warning lamp or message
- Loss or reduction of ABS/ESC functionality (limited braking/traction control)
- Incorrect or implausible hydraulic pressure reading on scanner
- Possible altered brake feel or warning buzzer
What to check
- Read and record DTC(s) and freeze frame data with a scan tool
- Check battery voltage and charging system (stable ~12–14.5 V)
- Visual inspection of sensor, connectors and wiring for damage, corrosion or water ingress
- Verify related fuses and power/ground circuits
- Check for other ABS/vehicle network codes (U-codes) that indicate module communication issues
- Monitor live brake pressure PID while operating brake pedal (with safe, stationary test or during low-speed test)
Signal parameters
- Sensor supply (reference) typical: ≈5.0 V (verify with scan tool/voltmeter)
- Signal voltage range (typical linear sensor): ≈0.5–4.5 V depending on pressure (0 to max system pressure)
- At rest (no hydraulic pressure) expect low-end signal — typically 0.5–1.0 V (varies by design)
- At high brake pressure expect high-end signal — typically 3.5–4.5 V (varies)
- Sensor return/ground: low resistance to chassis ground (
Diagnostic algorithm
- Connect a capable scan tool; read/record all DTCs and freeze frame. Note if code is current or intermittent.
- Perform a visual inspection of harnesses and connectors at the sensor and ABS module. Look for corrosion, damage, pin push-out, water intrusion or crushed wiring.
- With key ON (engine off) verify sensor reference voltage at the connector: should be ~5 V (manufacturer-specific). If missing, trace to fuse/ECU supply.
- Backprobe sensor signal wire and monitor voltage while slowly applying brake pressure (or have assistant press pedal while stationary/low speed). Verify the signal changes smoothly and within expected range. Look for jumps, open-circuit (~0 V) or stuck values.
- Check sensor ground continuity between sensor ground pin and vehicle chassis and module ground. Repair high-resistance grounds.
- Wiggle test harness/connector while monitoring live data to reproduce fault (intermittent wiring).
- If wiring and supply/ground are good but signal abnormal or non-responsive, disconnect sensor and measure sensor resistance per service data (or compare to a known-good sensor). Replace sensor if out of spec.
- If sensor bench/ohm tests pass and wiring is good, suspect ABS/brake control module fault; confirm with module-specific tests (reprogramming or swap with known-good module where applicable).
- After repair, clear codes, cycle ignition and perform functional test and road test to confirm no recurrence. Re-scan for pending codes.
- Document findings and repairs.
Likely causes
- Damaged or corroded sensor connector/wiring
- Failed pressure sensor element
- Open/short to battery or ground on sensor signal or supply
- Poor sensor ground or module supply fault
Fault status
Status
Brake hydraulic pressure sensor circuit malfunction — signal, supply or communication fault detected.
Repair difficulty: Medium
Diagnostic time: 1.0-2.5 hours
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