OBD-II Code C0024: Brake Assist System Fault

What Does C0024 Mean?

Code C0024 indicates the Brake Control Module detected a fault in the electronic brake assist system, which helps you stop faster in an emergency. Depending on the vehicle, this code points to a failed brake pressure solenoid, a faulty wheel speed sensor, corrupted control module software, or damaged wiring.

Technical definition: The SAE/ISO definition for C0024 varies by manufacturer. For modern General Motors vehicles, it is defined as 'Brake Pedal Simulator Separating Solenoid Valve' or 'Brake Pedal Feedback Pressure Solenoid Circuit Malfunction'. For Subaru, it is defined as 'Front Left ABS Sensor Signal Malfunction'. A specific symptom byte, C0024 5A on GM vehicles, points directly to an 'Internal Electronic Failure', confirming a software or module fault.

Can I Drive With C0024?

No — Do Not Drive. Do not drive. Your vehicle's Anti-lock Braking (ABS) and Stability Control systems are disabled, significantly increasing stopping distances and skidding risk. On many GM vehicles, this code indicates a total loss of power brake assist ('Push-Thru' mode), increasing stopping distances by 2 to 3 times. Tow the vehicle to a repair facility.

Common Causes

• Brake Control Module Software/Calibration Error (Very Common) — On 2019-2020 GM trucks and SUVs, a software glitch within the Brake System Control Module (BSCM) incorrectly calculates solenoid temperature, triggering the fault. The hardware is fine; the fix is a dealer software update.
• Faulty Wheel Speed Sensor (Common) — A dirty, damaged, or failed wheel speed sensor cannot report wheel speed to the ABS module. On Subaru models, C0024 specifically points to the front left sensor.
• Wiring Harness or Connector Damage (Common) — Corrosion on connectors, broken pins, or chafed wires interrupt the sensor signal. This is a known issue on Ford F-150s and GM trucks (where loose grounds per TSB 23-NA-201 cause loss of brake assist).
• Low System Voltage (Common) — Electronic brake systems require stable voltage. A weak battery or failing alternator causes modules to malfunction and set false codes, especially during engine cranking.
• Damaged or Misaligned ABS Tone Ring (Less Common) — The metallic ring on the wheel hub that the speed sensor reads cracks or misaligns. The sensor works correctly but receives an erratic signal from the damaged ring.
• Failing ABS/Brake System Control Module (Less Common) — An internal hardware failure in the control module, such as a bad microprocessor or failed driver transistor, sets this code even if all external parts function perfectly.
• Faulty Brake Pedal Position Sensor (Rare) — If the brake pedal position sensor fails or sends erratic signals, the module cannot correlate pedal movement with brake system pressures, setting the code.

Symptoms

• ABS Light On — The Anti-lock Braking System (ABS) warning light illuminates on the dashboard.
• Traction Control/Stability Control Light On — The TCS or ESC light illuminates, indicating these safety systems are disabled.
• Change in Brake Pedal Feel — The brake pedal feels unusually hard to press (common on GM), soft and spongy, or travels further to the floor than normal.
• Service Brake Assist Message on DIC — The Driver Information Center displays a 'Service Brake Assist' or similar warning message.
• Increased Stopping Distance — Without ABS and brake assist, the vehicle takes significantly longer to stop during emergencies.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this code.

Common Fixes & Costs

• Reprogram Brake System Control Module (BSCM) — Parts: $0, Labor: $150-$250, ~1.0 hr book time (Professional)
• Replace Wheel Speed Sensor — Parts: $40-$150, Labor: $100-$200, ~1.0 hr book time (DIY)
• Repair Wiring Harness or Connector — Parts: $10-$50, Labor: $100-$250, ~1.5 hr book time (Intermediate)
• Replace Brake Pedal Position Sensor — Parts: $40-$150, Labor: $75-$200, ~0.8 hr book time (Intermediate)
• Replace ABS Control Module — Parts: $700-$2,500, Labor: $200-$400, ~2.0 hr book time (Professional)

DIY vs Professional

• Reprogram Brake System Control Module (BSCM) — Beginner:
  Tools: GM MDI2 interface, GM Techline Connect subscription, stable power supply.
• Replace Wheel Speed Sensor — Beginner:
  Tools: Jack, jack stands, socket set, torque wrench, penetrating oil.
• Repair Wiring Harness or Connector — Beginner:
  Tools: Multimeter, wire strippers, heat shrink tubing, soldering iron or quality crimp connectors.
• Replace Brake Pedal Position Sensor — Beginner:
  Tools: Basic hand tools, Advanced OBD-II Scanner.
• Replace ABS Control Module — Beginner:
  Tools: Advanced scan tool, flare nut wrenches, pressure bleeder, manufacturer-specific programming tools.

What Happens If You Wait — Timeline

1. 0-2 weeks: ABS and Traction Control warning lights illuminate. ABS, stability control, and brake assist are disabled. Basic hydraulic brakes function, but stopping distances increase significantly. (MPG impact: 0%% · Added cost: $0)
2. 1-3 months: The driver becomes accustomed to the warning lights. The risk of losing control during a panic stop or on a slippery surface remains critically high. (MPG impact: 0%% · Added cost: Cost of an accident due to loss of ABS/ESC functionality.)
3. 3-12 months: If caused by a failing wheel bearing disrupting the speed sensor signal, the bearing degrades further, leading to severe noise, vibration, and eventual hub failure. (MPG impact: 1-5% (due to increased drag from a failing bearing)% · Added cost: $400 - $800 to replace the wheel hub assembly.)
4. 12+ months: Ignoring an internal ABS module fault causes complete electronic failure, making the vehicle undrivable and requiring full module replacement. (MPG impact: 0%% · Added cost: $1,500 - $3,000+ for a replacement ABS module and programming.)

Diagnosis Steps

1. Check for Technical Service Bulletins (TSBs)
   Search for your vehicle's make, model, year, and 'C0024 TSB'. For 2019-2020 GM vehicles, TSB 20-NA-073 dictates a software update. Also check TSB 23-NA-201 for loose grounds on GM trucks.
   Tools: Internet access (Beginner)
2. Read Freeze Frame & Symptom Byte Data
   Use an advanced OBD-II scanner to read freeze frame data. On GM vehicles, check for symptom byte '5A' (Internal Electronic Failure), which confirms the TSB software issue is the cause.
   Tools: Advanced OBD-II Scanner (Beginner)
3. Visually Inspect the Wheel Speed Sensor, Tone Ring, and Wiring
   Inspect the relevant wheel speed sensor (front left for Subaru). Check for damage, dirt, or metal debris. Inspect wiring for chafing. Carefully inspect the ABS tone ring on the hub for cracks or misalignment.
   Tools: Flashlight, Jack and Jack Stands (Beginner)
4. Test the Wheel Speed Sensor Signal (Graphing)
   Use an advanced scanner with live data graphing. Watch the signal from the suspect sensor while spinning the wheel. A good sensor shows a clean wave pattern; a bad sensor shows dropouts or erratic jumps.
   Tools: Advanced OBD-II Scanner, Jack and Jack Stands (Intermediate)
5. Check Live Data for Brake Pressure Sensor
   Monitor the live data PID for 'Brake Pressure'. With your foot off the pedal, it must read zero. If stuck at a high value or unresponsive to pedal presses, the internal pressure sensor has failed.
   Tools: Advanced OBD-II Scanner (Advanced)
6. Test Circuit Voltage and Ground
   Check for reference voltage (5V or 12V) and a solid ground at the sensor connector. With the component disconnected, check for supply voltage (11-14V) and continuity to ground at the solenoid connector.
   Tools: Multimeter (Intermediate)
7. Test Component Resistance and AC Voltage
   Measure internal resistance of passive sensors (typically 900-2,700 ohms) or solenoids (1-40 ohms). Next, set the multimeter to AC Volts, probe the passive sensor pins, and spin the wheel. It must generate at least 0.2-0.25 VAC. Do NOT test resistance on active sensors.
   Tools: Multimeter, Jack and Jack Stands (Advanced)
8. Perform Solenoid Actuation Test
   Use a high-end scan tool to command the brake assist solenoid to activate. Listen for a click. Alternatively, apply 12V directly to the solenoid's coil; if it clicks, the solenoid functions and the issue lies in the wiring or module.
   Tools: Advanced OBD-II Scanner, Power Probe or Jumper Wires (Professional)
9. Analyze Wheel Speed Sensor with Oscilloscope
   Connect an oscilloscope to the sensor wires. A healthy digital sensor produces a clean square wave; a passive sensor produces a sine wave. Distorted or missing waveforms definitively point to a sensor, tone ring, or wiring failure.
   Tools: Oscilloscope, Back-probe pins, Jack and Jack Stands (Professional)

When This Code Triggers (Freeze-Frame Conditions)

• Vehicle Speed: 0-15 mph (Triggers during the initial self-test at startup or low-speed braking maneuvers when sensor discrepancies first appear.)
• Brake Pedal Status: Applied (Detected when the brake pedal is pressed and the ABS module receives no response from a sensor or solenoid.)
• System Voltage: 11.5-14.5V (Triggers if system voltage drops during startup, as sensitive electronic brake modules require stable power.)
• Wheel Speed Sensor Data: Mismatch (Triggers when one wheel speed sensor reports a value inconsistent with the other three.)

Related Codes

• C0024 5A — A GM-specific symptom byte meaning 'Internal Electronic Failure'. Confirms TSB 20-NA-073 software update is the required fix, not a hardware replacement.
• C0035 — A generic code for a wheel speed sensor circuit fault. If present with C0024 on a GM vehicle, it indicates a separate wheel speed sensor problem requiring independent diagnosis.
• C0042 — Means 'Power Supply Voltage Failure'. Often appears with C0024 on Subarus following a dead battery or jump start. Diagnose the charging system first.
• C05D2 — Means 'Brake Master Cylinder Piston Excessive Travel'. Related to the GM eBoost system and mentioned in TSB 20-NA-073, reinforcing the likelihood of the software glitch.

Climate & Environmental Factors

• Cold Climates / Road Salt: Road salt and moisture attack wiring harnesses and connectors, leading to shorts or open circuits. This is the primary cause of sensor failure in 'Rust Belt' regions.
• High Humidity: Accelerates corrosion on exposed connector pins or compromised wire insulation, causing intermittent signal loss.

How to Talk to a Mechanic About This Code

Say this: "When calling for a diagnosis, be specific. For a 2019-2020 GM truck/SUV, say: 'I have a C0024 code and a hard brake pedal. Please check if my vehicle is covered by TSB 20-NA-073 for the BSCM software update before performing hardware diagnostics.' For a Subaru, say: 'My car has a C0024 code. This points to the front left wheel speed sensor circuit, so please start the diagnosis there.'"

This language signals you are an informed consumer. It directs the technician to the most probable cause, saving diagnostic time and preventing expensive misdiagnoses like replacing the brake booster.

Avoid saying:

• 'My ABS light is on, can you fix it?' (Too vague; invites expensive diagnostics.)
• 'Just do whatever it takes to fix it.' (Gives the shop a blank check.)
• 'My friend said it's probably a sensor.' (Let the shop follow the data-driven path you provided.)

Questions to ask before authorizing the repair:

• For a GM vehicle: Did you confirm my VIN is eligible for the TSB 20-NA-073 software update?
• If recommending hardware replacement: What specific tests ruled out the software issue or a wiring fault?
• For a non-GM vehicle: Can you show me the live data graph from the faulty wheel speed sensor compared to a good one?
• Is the recommended part OEM or aftermarket, and what is the warranty on parts and labor?

Where to Take It: Dealer vs Independent vs Chain

• Dealer:
  Best for: 2019-2020 GM vehicles requiring the dealer-only software update (TSB 20-NA-073)., Vehicles where the repair is covered by a powertrain, emissions, or CPO warranty., Complex module replacements requiring manufacturer-specific programming.
  Downsides: Highest labor rates, often 1.5-2x more than independent shops., May default to replacing entire assemblies rather than targeted wiring repairs. (Typical cost: +50% vs. baseline)
• Independent Shop: Best for non-GM vehicles or second opinions. For 2019-2020 GM models, use the dealer unless the independent shop explicitly confirms they have GM Techline Connect for the TSB reprogram.
  Best for: Out-of-warranty vehicles with non-software C0024 faults (e.g., Subaru wheel speed sensor, Ford wiring issue)., Owners seeking flexible repair options.
  Downsides: Quality and equipment vary widely., May lack expensive subscriptions required for module programming on newer cars. (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID for diagnosing C0024. The complexity and manufacturer-specific quirks of this code are beyond the scope of most chain shops.
  Best for: Simple, unrelated maintenance like oil changes or tires.
  Downsides: Technicians are unlikely to be aware of specific TSBs., High likelihood of misdiagnosing the GM software issue as a bad brake booster., Not equipped for module programming or in-depth electrical diagnostics. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the estimated repair cost exceeds 40-50% of your car's private-party value, seriously consider selling or trading in the vehicle.

• Car worth $28000, fix is $250: Fix it. A software update on a newer GM truck is a minor cost relative to the vehicle's value.
• Car worth $7000, fix is $300: Fix it. Replacing a wheel speed sensor on an older Subaru is well below the threshold.
• Car worth $4000, fix is $2200: Walk away. A $2,200 quote for an ABS module replacement exceeds 50% of the car's value. Get a second opinion, but prepare to sell 'as-is'.

What Scan Tool You Need for This Code

Minimum: A scanner that reads and clears ABS codes and displays live data streams for all four wheel speed sensors simultaneously.

A basic $20 OBD-II code reader only reads powertrain (P-codes) and cannot detect chassis codes like C0024.

Budget: FOXWELL NT630 Plus / ANCEL AD610 Pro (~$99) — Reads and clears ABS codes, graphs live wheel speed sensor data to identify faulty sensors, and offers limited bidirectional tests.

Mid-range: Autel MaxiCOM MK808 / Foxwell NT809 (~$380) — Reads GM-specific symptom bytes (like C0024 5A) and offers bidirectional controls for automated ABS bleed procedures. Cannot perform ECU reprogramming.

Professional: Autel MaxiSys MS906 / LAUNCH X431 Series (~$1200+) — Offers OE-level diagnostics and, combined with a J2534 passthrough device and manufacturer subscription, performs module reprogramming required by GM TSBs.

Rent vs buy: Some auto parts stores offer loaner scanners that read ABS codes, but capability varies. If you perform DIY repairs annually, buying a budget ABS-capable scanner is a worthwhile investment.

How to Clear the Code After You Fix It

1. Use an ABS-capable OBD-II scan tool to erase the fault code.
2. Drive the vehicle for 15 minutes above 15 mph to allow the ABS module to self-test and confirm the repair.
3. Verify the ABS warning light remains off.

Drive cycle (~15 minutes): Start the vehicle and drive for 10-15 minutes. Include speeds above 15 mph, gentle turns, and several smooth stops to allow the ABS module to verify all wheel speed sensors provide consistent data.

Readiness monitors affected: None. ABS faults (C-codes) do not affect emissions-related readiness monitors required for smog checks.

Watch out for:

• Using a basic OBD-II scanner incapable of reading or clearing ABS codes.
• Disconnecting the battery, which fails to clear the code from the ABS module's non-volatile memory.
• Clearing the code without fixing the underlying mechanical or electrical fault, causing immediate return.

Will This Fail Emissions / State Inspection?

No — by itself this code doesn't fail OBD inspection (but it can keep readiness monitors from setting, which causes a separate fail).

• California: An illuminated ABS light causes a failure of the mandatory vehicle safety inspection, even though it does not affect the OBD-II emissions plug-in test.
• New York: An illuminated amber ABS light is an advisory and does not cause an inspection failure. However, if the red 'BRAKE' warning light is also on, the vehicle fails.
• Texas: An illuminated ABS warning light is explicitly NOT a reason for safety inspection failure. The vehicle passes if standard service brakes work and the red 'BRAKE' light is off.

Most Commonly Affected Vehicles

• Chevrolet Silverado 1500 (2019-2020) — Extremely common due to a software glitch in the BSCM. Fixed by a dealer update per TSB 20-NA-073.
• GMC Sierra 1500 (2019-2020) — Shares the Silverado platform and is equally prone to the TSB 20-NA-073 software fault. Also check TSB 23-NA-201 for loose grounds.
• Subaru WRX / Impreza / Outback (2008-2015) — C0024 specifically points to a fault with the front left ABS wheel speed sensor signal. Shimming the sensor with a washer often fixes air gap issues.
• Ford F-150 (2015-2020) — Triggered by corroded wiring connectors to wheel speed sensors or a failing ABS module requiring Programmable Module Installation (PMI).
• Honda Odyssey (2018-2021) — Related to a faulty wheel speed sensor or internal failure of the VSA (Vehicle Stability Assist) modulator-control unit.
• Hyundai Santa Fe / iLoad (2013-2018) — Commonly caused by a damaged or detached ABS tone ring on the wheel hub, which gives a bad signal to a perfectly good sensor.
• Cadillac CT6 / XT5 / XT6 (2019-2020) — Included in GM's TSB 20-NA-073 for the brake assist software issue causing a hard brake pedal.
• Chevrolet Blazer / Acadia (2019-2020) — Included in GM's TSB 20-NA-073, susceptible to the brake control module software fault.

Manufacturer-Specific Notes

• General Motors (Chevy/GMC): On 2019-2020 trucks and SUVs, C0024 almost always indicates a software problem requiring a dealer reprogram per TSB 20-NA-073, covered under the 8-year/80,000-mile Federal Emissions Warranty.
• Subaru: C0024 specifically means 'Front Left ABS Sensor Signal Malfunction'. It points directly to the wheel speed sensor circuit, not a brake booster solenoid.
• Hyundai: The root cause is frequently a cracked or misaligned ABS tone ring on the wheel hub assembly, rather than a failed wheel speed sensor.
• Ford: Technicians must first inspect wiring harnesses near the wheels and along the frame, as corrosion mimics sensor or module failures.

Real Owner Stories

2019 GMC Sierra with hard brake pedal

Owner experienced 'Brake System Failure' and 'Service ESC' warnings, complete loss of power brake assist, and an extremely hard brake pedal.

Outcome: The dealership identified a known software issue and reprogrammed the Brake System Control Module (BSCM) per TSB 20-NA-073, resolving the problem.

Lesson: For 2019-2020 GM trucks, a hard pedal with C0024 is almost certainly the TSB 20-NA-073 software glitch. Insist the dealer checks for this update before authorizing expensive hardware replacements.

2014 Subaru WRX with multiple codes after knuckle replacement

After replacing the driver-side knuckle, the ABS activated during every stop, throwing codes C0024, C0042, and C0057.

Outcome: The owner resolved the issue by adding a washer to shim the sensor, fixing an incorrect air gap between the sensor and the tone ring.

Lesson: On Subarus, C0024 points directly to the front left wheel speed sensor circuit. If a new sensor fails to fix it, adjust the air gap with shims or inspect the ABS tone ring.

2020 GMC Sierra AT4 with misdiagnosed master cylinder

At 16,200 miles, the truck displayed 'Service brake assist'. The dealership diagnosed a leaking master cylinder and quoted over $3,400.

Outcome: The owner prepared to sell the vehicle due to frustration with the misdiagnosis.

Lesson: Reject immediate recommendations for expensive hardware replacement on newer GM trucks with C0024. A hard pedal is the classic symptom of the software flaw, frequently misdiagnosed as a failed booster or master cylinder.

2019 GMC Sierra with brake codes after battery replacement

Immediately after replacing the battery, the truck displayed 'Service ESC' and set codes P0606, C0021, and P25A2.

Outcome: The issue required a dealer-level diagnostic tool to clear the voltage-induced module lockup.

Lesson: On vehicles with electronic brake assist (eBoost), swapping a battery triggers complex faults in the Brake System Control Module. Voltage drops set codes requiring dealer tools to clear.

How to Prevent This Code From Triggering

• Perform brake fluid flush at recommended intervals (Every 2-3 years or 30,000 miles) — Brake fluid absorbs moisture over time. Contaminated fluid corrodes sensitive solenoids and valves inside the ABS module, causing mechanical failure.
• Clean, protect, and secure electrical connectors (During brake service or seasonal undercarriage washes) — Applying dielectric grease to wheel speed sensor connectors seals out moisture. Securing wiring harnesses prevents chafing. Washing away road salt prevents the corrosion that causes sensor signal failures.

Frequently Asked Questions

My mechanic wants to replace my brake booster for a C0024 code. Is this correct?

On a 2019-2020 GM vehicle, this is a common misdiagnosis. The hard pedal feel leads technicians to suspect a failed booster, but the actual cause is the software glitch described in TSB 20-NA-073. Insist on checking for the software update before authorizing hardware replacement.

What is the 'Push-Thru' fail-safe mode on my GM truck?

Push-Thru is a fail-safe mode triggered by faults like C0024 where electronic brake assist is completely disabled. You are left with a direct mechanical link to the brakes, requiring significantly more pedal effort and resulting in much longer stopping distances.

Can I perform the GM software update myself to fix code C0024?

No. Reprogramming the BSCM requires a GM Techline Connect subscription, a specific vehicle communication interface, and a stable power supply to prevent permanently damaging the module. Leave this job to a GM dealer.

Are there any recalls related to code C0024?

Yes. GM issued recall N192268090 for certain 2019 trucks because a software error disables ABS without warning. However, the more common fix for the hard pedal symptom is covered by TSB 20-NA-073, not a recall. Always check your specific VIN.

Can a bad ground wire cause code C0024?

Yes. On GM trucks, TSB 23-NA-201 notes that a loose ground connection causes a loss of brake assist and illuminates warning lights. Check this if the main software TSB does not apply.

Is replacing a wheel speed sensor a DIY job?

Yes. It typically involves removing the wheel, undoing one bolt, and unplugging a connector. However, sensors seize in the wheel hub on older cars in rust-prone areas, making removal difficult.

The shop quoted me $1,500 to fix code C0024. Is that a fair price?

It depends entirely on the repair. A new ABS control module costs over $1,000 for the part alone, plus labor and programming. However, a simple wheel speed sensor or wiring repair typically costs under $350.

Key Takeaways

• Code C0024 disables your vehicle's ABS and Stability Control systems, increasing stopping distances by up to 3 times.
• For 2019-2020 GM trucks and SUVs, a software glitch causes this code and is fixed by a dealer reprogram (TSB 20-NA-073), often covered under the 8-year/80,000-mile emissions warranty.
• On Subaru models, C0024 specifically indicates a failure in the front left wheel speed sensor circuit, requiring a $140 to $350 sensor replacement or wiring repair.
• Stop driving immediately and tow the vehicle if the brake pedal becomes extremely hard to press, as this indicates a complete loss of power brake assist.