C0138 is a chassis trouble code that usually points to a problem in the ABS (anti-lock brake system) or electronic brake control system, most often related to low or incorrect brake system voltage. When this code sets, your ABS module is basically telling you it doesn’t like the power it’s getting. You might see an ABS light, traction control light, or stability control warning on the dash, and sometimes feel changes in braking behavior. Understanding what C0138 means, how it behaves, and how to fix it can save you time, money, and stress.
What Does C0138 Mean?
C0138 is a chassis DTC that typically translates to “ABS/TCS System Voltage Low” or “Brake System Voltage Malfunction,” depending on the manufacturer. It’s most commonly seen on GM (Chevrolet, GMC, Buick, Cadillac), but can appear on other brands that use similar code structures.
In simple terms, the ABS or electronic brake control module (EBCM) is detecting voltage that’s outside of its normal operating range. That could be from a weak battery, poor ground, corroded wiring, or an internal fault in the module itself. When this happens, the module may partially or completely disable ABS and traction control to protect itself.
Quick Reference
- Code: C0138
- System: Chassis – ABS / Traction / Stability control
- Typical definition: ABS or brake system voltage low / malfunction
- Risk level: Moderate – braking still works, but ABS/traction may be disabled
- Fix difficulty: Moderate – electrical testing often required
Real-World Example / Field Notes
In the shop, I see C0138 a lot on full-size GM trucks and SUVs. A common scenario: you start getting an intermittent ABS light and “Service Brake System” or “Service Stabilitrak” message. Sometimes it happens after a cold start or when you hit a bump. We hook up a scan tool, pull C0138, and then check live data for ABS supply voltage. Often we find a weak battery dropping under load, or a corroded ground cable near the frame. Clean the grounds, replace the failing battery, clear the code, and the ABS light stays off.
On other vehicles, the story can be different. I’ve had a case where the wiring harness to the ABS module rubbed through on a bracket, causing intermittent low voltage to the module. The driver complained of random traction control activation and warning lights. A quick visual inspection and a multimeter test confirmed the wiring issue. A simple harness repair and proper routing fixed the problem without needing a new module.
Symptoms of C0138
- ABS warning light on: The most common sign, often accompanied by a chime or message.
- Traction or stability light: “TCS,” “ESC,” or “Stabilitrak” lights may turn on or flash.
- Loss of ABS function: Under hard braking, wheels may lock instead of pulsing.
- Longer stopping distance: Especially on wet, snowy, or loose surfaces.
- Intermittent warnings: Lights that come and go, often with bumps or cold starts.
- Battery/charging issues: Slow cranking, dim lights, or other electrical oddities.
- Stored chassis codes: Other ABS or stability control codes may appear alongside C0138.
Common Causes of C0138
Most Common Causes
- Weak or failing battery: Low voltage during cranking or under load can trigger C0138.
- Poor battery connections: Corroded or loose battery terminals or cable ends reduce voltage to the ABS module.
- Bad grounds: Rusty or loose ground straps between battery, engine, body, and frame are a frequent culprit.
- Charging system problems: Alternator not charging properly, causing voltage drops or spikes.
- Corroded ABS power feed: Fuse box or connector corrosion on the ABS power supply circuit.
Less Common Causes
- Damaged wiring harness: Chafed, pinched, or broken wires feeding the ABS/EBCM.
- Faulty ABS/EBCM module: Internal failure causing incorrect voltage readings or internal short.
- Aftermarket accessories: Poorly wired stereos, light bars, or remote starts causing voltage instability.
- Incorrect battery type: Wrong size or low-capacity battery not meeting vehicle requirements.
- Water intrusion: Moisture in connectors or the module itself leading to corrosion and resistance.
Diagnosis: Step-by-Step Guide
Before you start, it helps to have a good scan tool capable of reading ABS codes and live data, a digital multimeter (DMM), basic hand tools, and battery/terminal cleaning supplies. A wiring diagram for your specific vehicle is also very helpful.
- Confirm the code: Connect a scan tool, read all modules, and verify C0138 is present. Note any related ABS or powertrain codes.
- Check battery health: With the engine off, measure battery voltage at the terminals. You want around 12.4–12.7V. Under 12.2V suggests a weak battery.
- Inspect battery connections: Look for corrosion, loose clamps, or damaged cables. Wiggle the terminals slightly; they should be tight and solid.
- Load test the battery: Use a battery tester or have a parts store load test it. A battery can show good voltage but fail under load, causing voltage dips that trigger C0138.
- Check charging system: Start the engine and measure voltage at the battery. Normal range is about 13.5–14.7V. If it’s outside that range, suspect the alternator or voltage regulator.
- Inspect grounds and power feeds: Locate main grounds (engine to body, battery to body, frame grounds). Clean and tighten them. Check ABS/EBCM power fuses and relays, and look for corrosion in the fuse box.
- Measure voltage at the ABS module: With a wiring diagram, back-probe the power and ground pins at the ABS/EBCM connector. Compare the reading to battery voltage. A significant drop indicates wiring or connector issues.
- Check for damaged wiring: Follow the harness from the battery and fuse box to the ABS module. Look for rubbed-through spots, sharp bends, or areas near moving parts and hot exhaust.
- Use scan tool data: Many scan tools show ABS supply voltage in live data or Mode $06. Compare this value to your multimeter reading. If the module reports low voltage while the actual voltage is normal, the module may be failing.
- Clear and retest: After repairs or cleaning, clear the code and test-drive. Watch ABS voltage and see if C0138 returns under similar conditions (cold start, bumps, heavy braking).
Pro tip: Always test voltage under real load conditions. A circuit can look fine with a simple key-on check, but wiggle the harness or turn on multiple electrical loads (headlights, blower, rear defogger) and a weak connection will often reveal itself.
Possible Fixes & Repair Costs
Repairs for C0138 range from simple cleaning to module replacement. The exact cost depends on what you find during diagnosis, your vehicle model, and whether you do the work yourself or use a shop.
- Clean/tighten battery terminals and grounds: Often under $50 in parts, or $0 if you already have supplies. A shop may charge 0.5–1.0 hour labor.
- Battery replacement: Typically $120–$300 depending on battery type and brand, plus installation if done at a shop.
- Alternator/charging system repair: Parts and labor can run $300–$800 depending on vehicle and alternator location.
- Wiring repair: Fixing a damaged harness section may cost $100–$400, more if the harness is buried or requires extensive disassembly.
- ABS/EBCM replacement: One of the more expensive fixes. New or reman modules often run $400–$1,000+ plus programming and labor.
In general, you’re looking at a typical repair range of about $100–$900 for most C0138 cases. Simple connection issues are on the low end; module or charging system replacement pushes you toward the high end. Labor rates, parts availability, and dealer vs. independent shop all affect the final price.
Can I Still Drive With C0138?
In most cases, you can still drive with C0138, but you need to understand the risk. Your basic hydraulic brakes still work, so the car will stop, but ABS, traction control, and stability control may be disabled or unreliable. That means in an emergency stop or on slippery roads, the vehicle can skid or lose traction more easily.
If the ABS and traction lights are on solid and the vehicle feels normal under light braking, you can usually drive short distances to get it checked. However, if you notice longer stopping distances, pulsing that feels abnormal, or other electrical issues (dim lights, hard starting), you should address it as soon as possible and avoid high-speed or heavy-traffic driving until it’s fixed.
What Happens If You Ignore C0138?
If you ignore C0138, you’re essentially driving without the safety net of ABS and stability control. Over time, the underlying electrical issue can get worse, potentially affecting other systems that share power or ground paths. A weak battery or bad alternator can leave you stranded, and a failing ABS module can lead to more expensive repairs if corrosion or internal damage spreads. It’s not an immediate “park it now” emergency, but it’s not something you should put off for months either.
Related Codes
- C0194 – Chassis Wheel Speed Signal Plausibility
- C0193 – Traction Control Torque Request Signal Range/Performance
- C0192 – Chassis Wheel Speed Signal Plausibility
- C0191 – Traction Control Torque Request Signal Low
- C0190 – Traction Control Torque Request Signal
- C0189 – Brake Wheel Speed Signal Plausibility
- C0188 – Stop Lamp Switch Circuit Range/Performance
- C0187 – Brake Wheel Speed Signal Plausibility
- C0186 – Chassis Wheel Speed Sensor Circuit Fault
- C0185 – Chassis Wheel Speed Signal Plausibility
Key Takeaways
- C0138 points to an ABS or brake system voltage problem, not a basic hydraulic brake failure.
- Most causes are electrical: weak battery, poor connections, bad grounds, or charging system issues.
- Ignoring the code can leave you without ABS/traction control when you need it most.
- Diagnosis starts with the basics: battery, alternator, fuses, and grounds before condemning the ABS module.
- Repair costs vary widely, but many fixes are relatively affordable if caught early.
Vehicles Commonly Affected by C0138
C0138 is especially common on GM products that use this chassis code format. You’ll often see it on:
- Chevrolet: Silverado, Tahoe, Suburban, TrailBlazer, Impala, Malibu.
- GMC: Sierra, Yukon, Envoy.
- Buick: LaCrosse, Enclave, Regal.
- Cadillac: Escalade, CTS, SRX.
It can also appear on other manufacturers that share similar ABS module strategies, particularly larger SUVs, trucks, and crossovers that put more load on the electrical system. Age, rust-prone climates, and vehicles with lots of aftermarket electrical accessories tend to see this code more often.
FAQ
Can I clear C0138 myself and keep driving?
You can clear C0138 with a capable scan tool, but if the root cause isn’t fixed, the code will usually return. Clearing it temporarily turns off the warning lights, but the ABS module will see the same low-voltage condition again and set the code. It’s fine to clear it after repairs or to confirm an intermittent issue, but don’t treat clearing the code as the repair.
Is C0138 always caused by a bad ABS module?
No, a bad ABS or EBCM module is actually one of the less common causes. Most of the time, C0138 is caused by a weak battery, poor connections, corroded grounds, or charging system problems. You should rule out all power and ground issues before replacing the module, because modules are expensive and often require programming.
How do I know if my battery is causing C0138?
If your battery is old, the engine cranks slowly, lights dim at idle, or voltage drops below about 12.2V with the engine off, the battery is suspect. A proper load test is the best way to confirm it. If C0138 only appears during cold starts or when you use a lot of electrical accessories, that’s another strong hint that the battery or charging system is the problem.
Can C0138 make my brakes feel spongy or soft?
<pC0138 itself doesn’t usually cause a soft pedal. It mainly affects the electronic side of the brake system (ABS, traction, stability), not the hydraulic pressure. A spongy pedal is more often due to air in the brake lines, worn pads, or a hydraulic leak. However, when ABS is disabled, the pedal “feel” can change slightly under hard braking, which some drivers interpret as different or less controlled.
Is it safe to drive on the highway with C0138 set?
Mechanically, the car will still stop, but you’re driving without a fully functional ABS and stability control system. On a dry, straight highway in light traffic, the risk is lower, but if you need to brake hard or swerve, you won’t have the same level of electronic assistance. If you must drive on the highway, keep extra distance, avoid aggressive maneuvers, and plan to repair the issue as soon as possible.