C0138 is a manufacturer-specific chassis/ABS diagnostic trouble code. Its exact definition and diagnostic procedure can vary depending on the vehicle manufacturer, ABS system, and model year.
On applicable GM vehicles, C0138 is associated with a Base Brake System Pressure Circuit Low condition. The ABS/EBCM has detected a pressure-sensor signal that is lower than the expected electrical range.
C0138 does not automatically prove that actual hydraulic brake pressure is too low. A faulty pressure sensor, wiring problem, short to ground, poor reference voltage, connector fault, sensor-ground problem, or an actual hydraulic brake condition can cause the code.
C0138 Fault Code Description
C0138 = Base Brake System Pressure Circuit Low.
Code type: Manufacturer-specific Chassis/ABS code.
Primary system: Base brake pressure monitoring, ABS/EBCM, and related electronic braking-control functions.
The EBCM monitors the brake-pressure signal and compares it with expected operating values. When the electrical input from the pressure circuit is below the expected range, C0138 may be stored.
What Does C0138 Mean?
C0138 means that the ABS/EBCM has detected a low electrical signal from the base brake-system pressure circuit.
The word Low primarily describes the electrical pressure-sensor signal. It should not automatically be interpreted as proof that the hydraulic brake pressure is low.
The electrical circuit and the actual hydraulic brake condition should therefore be tested separately during diagnosis.
Symptoms of C0138
- ABS warning light.
- Brake warning light.
- Traction Control warning light.
- Stability Control warning light where equipped.
- Service ABS message.
- Service Traction System message where equipped.
- ABS functionality may be disabled.
- Traction control may be disabled.
- Stability-control functions may be disabled.
- Stored ABS/EBCM diagnostic trouble code.
The exact symptoms depend on the vehicle, ABS system, and severity of the underlying fault.
Common Causes of C0138
- Faulty brake-pressure sensor.
- Pressure sensor signal circuit shorted to ground.
- Open or damaged reference-voltage circuit.
- Incorrect sensor reference voltage.
- Poor sensor ground.
- Damaged pressure-sensor wiring.
- Corroded or loose electrical connector.
- High resistance in the pressure circuit.
- Actual low hydraulic brake pressure.
- Brake-system hydraulic problem.
- Internal EBCM fault.
How to Diagnose C0138
1. Scan the ABS/EBCM
Use an ABS-capable scan tool to communicate with the EBCM. Confirm C0138 and check for additional ABS, pressure-sensor, voltage, communication, or brake-system codes.
Record freeze-frame data when available before clearing the code. Additional codes may help identify whether the problem is electrical or hydraulic.
2. Check Brake Pressure Live Data
Monitor the available brake-pressure data with the scan tool. Observe the pressure value while the brake pedal is applied and released.
A pressure value that remains abnormally low, stays fixed, or does not respond appropriately should be investigated.
3. Inspect the Pressure Sensor Connector
Inspect the pressure sensor connector for corrosion, water intrusion, loose terminals, damaged pins, contamination, or poor terminal tension.
4. Check Reference Voltage
Use the vehicle-specific wiring diagram to identify the sensor reference-voltage circuit. Verify the reference voltage according to the manufacturer's specifications.
A missing or incorrect reference voltage can cause a low pressure-sensor signal.
5. Check Sensor Ground
Test the pressure-sensor ground circuit for excessive resistance, an open circuit, or poor electrical contact.
A poor ground can alter the sensor output and cause an incorrect low-input condition.
6. Test the Pressure Signal Circuit
Check the signal circuit for a short to ground, open circuit, excessive resistance, damaged insulation, or intermittent connection.
Always verify the correct connector pinout for the exact vehicle and ABS system before applying test voltage.
7. Check Actual Brake Pressure
If the electrical circuit passes inspection, determine whether the actual hydraulic brake pressure is also below specification.
An actual low-pressure condition should be investigated separately from an electrical low-input fault.
8. Check the EBCM
If the sensor, wiring, connector, reference voltage, ground, and hydraulic system test correctly, further EBCM diagnosis may be required.
Do not replace the EBCM based only on the presence of C0138.
C0138 Diagnostic Signal Flow
C0138 GM
On applicable GM vehicles, C0138 is associated with a Base Brake System Pressure Circuit Low condition.
The EBCM may set the code when the electrical pressure-sensor signal is below the expected operating range.
Diagnosis should include the pressure sensor, connector, wiring, reference voltage, sensor ground, live data, and actual brake-pressure condition where appropriate.
C0138 Cadillac
On applicable Cadillac vehicles using the relevant GM ABS/EBCM system, C0138 can indicate a Base Brake System Pressure Circuit Low condition.
Inspect the pressure sensor and connector, verify the reference voltage and ground, and test the pressure signal circuit before replacing components.
The exact sensor location, connector pinout, pressure specifications, and diagnostic procedure depend on the Cadillac model and ABS configuration.
C0138 Chevrolet
On applicable Chevrolet vehicles, C0138 may indicate a low electrical signal in the base brake-system pressure circuit.
The exact meaning and diagnostic procedure should be confirmed using service information for the specific Chevrolet model, year, and ABS system.
Is C0138 Serious?
C0138 should not be ignored. A fault in the brake-pressure monitoring circuit can affect ABS, traction control, or stability-control functions.
The basic hydraulic brakes may continue to operate, but electronic braking assistance may be unavailable while the fault is active.
If the brake pedal feels abnormal, braking performance changes, or a brake-fluid leak is present, the vehicle should not be driven until the braking system has been inspected.
Can I Drive With C0138?
Limited driving may be possible if the normal braking system operates correctly and there are no signs of hydraulic brake failure.
However, ABS, traction-control, or stability-control functions may be disabled. The problem should be diagnosed as soon as possible.
C0138 Repair Cost
Repair costs depend on the actual cause and the specific vehicle.
- Diagnostic testing: approximately $50–$150+.
- Wiring or connector repair: approximately $50–$300+.
- Pressure sensor repair/replacement: approximately $100–$500+.
- Brake hydraulic-system repair: approximately $150–$700+.
- EBCM repair or replacement: approximately $500–$1,500+.
These are general estimates only. Actual costs vary according to the vehicle, parts availability, labor rates, and failed component.
Common Repair Mistakes
Replacing the Pressure Sensor Immediately
Do not replace the pressure sensor simply because C0138 is stored. An electrical short, wiring problem, poor connector, reference-voltage issue, or ground fault can produce the same code.
Assuming Low Signal Means Low Brake Pressure
C0138 is an electrical low-input code. The pressure signal can be low even when the actual hydraulic brake pressure is normal.
Ignoring the Sensor Ground
A poor sensor ground can change the pressure-sensor output and cause an incorrect low-input condition.
Using the Wrong Pinout
Brake-pressure sensor wiring can differ between GM vehicles and ABS system generations. Always verify the correct connector pinout before testing the circuit.
Related Codes
- C0131 – ABS/TCS System Pressure Circuit Malfunction
- C0132 – ABS/TCS System Pressure Circuit Range/Performance
- C0133 – ABS/TCS System Pressure Circuit Low
- C0134 – ABS/TCS System Pressure Circuit High
- C0135 – ABS/TCS System Pressure Circuit Open
- C0136 – Base Brake System Pressure Circuit Malfunction
- C0137 – Base Brake System Pressure Circuit Range/Performance
- C0138 – Base Brake System Pressure Circuit Low
- C0139 – Base Brake System Pressure Circuit High
- C0140 – Base Brake System Pressure Circuit Open
FAQ About C0138
What does C0138 mean?
C0138 means Base Brake System Pressure Circuit Low on applicable GM ABS systems. The EBCM has detected a pressure-circuit signal below the expected electrical range.
What causes C0138?
Possible causes include a faulty pressure sensor, short to ground, damaged wiring, connector problems, incorrect reference voltage, poor sensor ground, an actual hydraulic brake problem, or an EBCM fault.
What does C0138 mean on Cadillac?
On applicable Cadillac vehicles, C0138 can indicate a Base Brake System Pressure Circuit Low condition. The pressure sensor, wiring, connector, reference voltage, ground, and hydraulic system should be checked as appropriate.
What does C0138 mean on Chevrolet?
On applicable Chevrolet vehicles, C0138 may indicate a low electrical signal from the base brake-system pressure circuit. The exact diagnostic procedure depends on the vehicle and ABS configuration.
Does C0138 mean the pressure sensor is bad?
Not necessarily. Wiring, connector, reference voltage, sensor ground, hydraulic pressure, and the EBCM can also cause C0138.
Can I drive with C0138?
Basic braking may remain available, but ABS, traction-control, or stability-control functions may be disabled. The fault should be diagnosed promptly.
How do I fix C0138?
Start by scanning the ABS/EBCM, checking brake-pressure live data, inspecting the pressure sensor and connector, testing reference voltage and ground, and checking the signal circuit. Verify actual hydraulic brake pressure before replacing components.
Final Result
C0138 is a manufacturer-specific chassis/ABS diagnostic trouble code. On applicable GM systems, including certain Cadillac and Chevrolet applications, it is associated with a Base Brake System Pressure Circuit Low condition.
The most reliable diagnosis is to scan the ABS/EBCM, monitor pressure live data, inspect the pressure sensor and connector, verify reference voltage and ground, test the signal circuit, and check the actual hydraulic brake condition where required.
Always confirm the exact C0138 definition, wiring, and diagnostic specifications for the specific vehicle and ABS system.