C0649

Code

Generic C — Chassis

Brake Pressure Sensor M Circuit/Open

Brand: Generic

Type: C — Chassis

Views: UK: 30 EN: 51 RU: 29

AI status

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Page language: EN

Causes

Open or broken wiring in the brake pressure sensor M circuit
Corroded, loose, or damaged sensor connector
Failed brake pressure sensor (internal open)
Short to ground or short to power on the sensor harness
Blown fuse or lost reference power/ground for the sensor circuit
Faulty ABS/BCM module or internal driver fault

Symptoms

ABS warning lamp or BRAKE warning lamp illuminated
Stored chassis/ABS fault(s) and possible limp ABS mode
Brake pressure reading missing or showing open on scan tool
Loss of ABS/traction control functionality (system may revert to non-ABS braking)
No obvious change to pedal effort in many vehicles (primary brakes often still work)

What to check

Retrieve freeze frame and live data with a scan tool; confirm code and check brake pressure sensor M live values
Visual inspection of sensor connector and wiring for damage, corrosion, or water intrusion
Backprobe sensor connector with ignition ON to check reference power and ground
Measure sensor signal voltage at rest and while applying brake pressure (with scan tool or pressure gauge where applicable)
Perform continuity and resistance check between sensor connector and ABS/BCM harness connector
Wiggle test wiring while observing live data/or resistance to reproduce fault

Signal parameters

Typical sensor output: analog 0–5 V signal (varies by manufacturer); at zero pressure often ~0.5–1.0 V, increases with pressure toward ~4.0–4.5 V
Reference/power supply: commonly a 5 V reference or switched supply from ABS/BCM (verify vehicle-specific value)
Open-circuit result: infinite resistance/OL on ohmmeter and signal shows no voltage or scanner shows '—' or 'open'
Short-to-ground: signal near 0 V; short-to-battery: signal near battery voltage
Scan tool: brake pressure channel will report invalid, zero, or erratic values when circuit is open

Diagnostic algorithm

Read and record all stored codes and freeze-frame data. Note if multiple brake/ABS codes are present.
Visual inspection: check sensor connector, pins and wires for corrosion, bent pins, loose terminals, or damage. Repair simple connector issues and clear codes to test.
With ignition ON (engine off), backprobe the sensor connector: verify reference power (typically ~5 V) and ground presence. If reference missing, trace to fuse/module ground.
Measure sensor signal voltage at the connector while an assistant applies the brake pedal (or use a diagnostic brake pressure function). Expect the signal to change with pressure. If signal is constant/open, suspect wiring or sensor.
Check continuity/resistance between the sensor connector and the ABS/BCM connector pins. An open or very high resistance indicates a harness fault; repair wiring as needed.
Inspect wiring routing for chafing/contact points near suspension or steering components and repair or replace damaged sections.
If wiring and connectors are good but signal remains open, replace the brake pressure sensor and retest.
If sensor replacement does not clear the fault and wiring is confirmed good, suspect ABS/BCM input driver fault. Verify module grounds and fuses, perform module tests per manufacturer, and replace module only after exhaustive wiring/sensor checks.
After repairs, clear codes and perform road test or pressure tests to confirm proper operation and that code does not return.

Likely causes

Corroded/loose connector at the pressure sensor
Damaged harness (pinched, chafed, or broken) near suspension/brake components
Failed pressure sensor element
Connector contamination (water, road salt) causing open contact
ABS module input driver fault (less common)

Fault status

⚠️ Status

Brake Pressure Sensor M Circuit/Open — Open or very high resistance detected in the Brake Pressure Sensor M circuit. ABS/traction control may be disabled. Check sensor, connector, wiring, and module inputs.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours