C0120 is a chassis diagnostic trouble code associated with the ABS (Anti-Lock Braking System). On many GM applications, C0120 is defined as Pump Motor Relay Circuit Open. However, the exact meaning of C0120 can vary by vehicle manufacturer, ABS module, and model year.
Because C0120 is a chassis code, it is normally stored in the ABS, Electronic Brake Control Module (EBCM), or related brake control system rather than the engine control module. Always confirm the exact manufacturer-specific definition before beginning repairs.
Fault Code Description
C0120 = Pump Motor Relay Circuit Open on applicable GM vehicles.
The code generally indicates that the ABS control module has detected an open or abnormal electrical condition in the circuit used to control the ABS pump motor relay. The exact circuit design depends on the vehicle.
An open circuit does not necessarily mean that the ABS pump motor itself has failed. The problem may be related to a fuse, relay, wiring, connector, power supply, ground circuit, ABS module, or an internal relay circuit.
What Does C0120 Mean?
C0120 means that the ABS control system has detected a fault associated with the pump motor relay circuit.
On applicable GM vehicles, the specific description is Pump Motor Relay Circuit Open. The Electronic Brake Control Module (EBCM) controls the ABS pump motor and monitors the electrical response of the circuit.
If the module cannot detect the expected electrical condition when the pump motor is commanded, C0120 may be stored.
C0120 ABS Pump Motor System
The ABS pump motor is part of the hydraulic braking system. During ABS operation, the pump helps maintain or restore hydraulic pressure while the system rapidly controls brake pressure at individual wheels.
Symptoms of C0120
- ABS warning light.
- Traction Control warning light.
- Stability Control warning light.
- Service Brake or related warning message.
- ABS system may become unavailable.
- Traction control may be disabled.
- Electronic stability control may be disabled.
- ABS pump may fail to activate when commanded.
- Additional ABS-related trouble codes may be stored.
The exact symptoms depend on the vehicle and the strategy used by its ABS control module.
Common Causes of C0120
- Open ABS pump motor relay circuit.
- Failed ABS pump motor relay.
- Internal EBCM relay circuit failure.
- Blown ABS fuse.
- Damaged ABS wiring.
- Open power supply circuit.
- Poor electrical connection.
- Corroded ABS connector.
- Poor ground connection.
- Damaged terminals.
- Faulty ABS pump motor.
- Internal ABS/EBCM failure.
- Low system voltage.
How to Diagnose C0120
1. Scan the ABS Module
Use an ABS-capable scan tool to retrieve C0120 and any additional chassis or brake-system codes.
Record the stored and pending codes before clearing them. Additional ABS codes can help determine whether the problem is electrical, hydraulic, or module-related.
2. Check the ABS Fuse
Inspect the ABS pump fuse and related power fuses. A blown fuse can interrupt power to the pump motor circuit.
Do not simply replace a blown high-current fuse without determining why it failed. A shorted pump motor or damaged wiring can cause the fuse to blow again.
3. Inspect the ABS Connector
Inspect the ABS/EBCM connector for corrosion, water intrusion, damaged terminals, loose connections, or signs of overheating.
4. Check Power and Ground
Verify battery voltage and the power and ground circuits supplying the ABS module and pump motor.
A voltage-drop test is useful when a circuit appears to have power but cannot supply the required current under load.
5. Test the Pump Motor Circuit
Using the vehicle-specific wiring diagram, identify the ABS pump motor power, ground, relay and control circuits.
Check for an open circuit, excessive resistance, short circuit, damaged wiring, or poor terminal contact.
6. Test the ABS Pump Motor
If the electrical supply and control circuits are correct, the ABS pump motor should be evaluated according to the manufacturer's diagnostic procedure.
Do not apply battery voltage directly to an ABS pump or module unless the manufacturer's service procedure specifically permits the test.
7. Test the ABS/EBCM
If the external wiring, fuse, power supply, ground and pump motor circuits pass testing, an internal EBCM relay or control-circuit fault may need to be investigated.
C0120 Diagnostic Signal Flow
C0120 Electrical Testing
Electrical testing should be performed with the vehicle-specific wiring diagram. The technician should identify the pump motor relay circuit, power supply, ground circuit and EBCM control circuit before measuring voltage or resistance.
A basic diagnostic sequence is:
- Check battery voltage.
- Check ABS fuses.
- Check relay operation where applicable.
- Check ABS module power supply.
- Check ABS module grounds.
- Check pump motor wiring.
- Check connector condition.
- Perform voltage-drop testing.
- Test the pump motor according to OEM specifications.
- Evaluate the EBCM if all external circuits pass.
C0120 Oscilloscope Testing
An oscilloscope can be useful when the ABS module or pump motor circuit is intermittent. Depending on the vehicle, the technician may monitor the pump motor command, relay control signal, supply voltage, or voltage response when the pump is activated.
A normal waveform should be compared with the manufacturer's specifications or a known-good vehicle. Do not assume that one universal waveform or voltage applies to every ABS system.
Is C0120 Serious?
Yes, C0120 can be serious because it may affect the ABS and related braking safety systems.
Depending on the vehicle, ABS, traction control, and stability-control functions may be disabled when the fault is active.
The conventional hydraulic brakes may still operate, but the vehicle may not provide the same level of electronic braking assistance during emergency or low-traction conditions.
Can I Drive With C0120?
Driving with an active C0120 should be minimized until the ABS system has been diagnosed.
If the ABS, traction-control, or stability-control warning lights are illuminated, drive carefully and avoid situations that require emergency braking or maximum traction until the fault is repaired.
If the vehicle has reduced braking performance, a brake warning light, abnormal brake pedal behavior, or other serious braking symptoms, stop driving and have the vehicle inspected.
C0120 Repair
The correct repair depends on the cause of the code.
- Repair an open wire.
- Repair damaged wiring.
- Clean or replace corroded terminals.
- Repair a poor ground connection.
- Replace a failed fuse.
- Replace the applicable relay where serviceable.
- Repair or replace the ABS pump motor if confirmed faulty.
- Repair or replace the EBCM when an internal relay or control fault is confirmed.
The ABS module or hydraulic assembly should not be replaced simply because C0120 is stored. Electrical testing should be completed first.
Common Repair Mistakes
Replacing the ABS Module Too Early
A blown fuse, damaged wiring, poor ground, or connector problem can produce symptoms similar to an internal ABS module failure.
Ignoring the Fuse
The ABS pump motor can require a high-current power supply. Always inspect the appropriate fuse and determine why it failed before replacing expensive components.
Testing With the Wrong Pinout
ABS connector pinouts vary significantly between manufacturers and model years. Always verify the correct wiring diagram before applying test voltage.
Assuming C0120 Has the Same Meaning on Every Vehicle
C0120 is not safe to interpret from the five-character code alone. Manufacturer specific information should be checked before selecting a diagnostic procedure.
C0120 on GM Vehicles
On applicable GM vehicles, C0120 is documented as Pump Motor Relay Circuit Open.
The EBCM controls the ABS pump motor and monitors the circuit. Depending on the specific system, the relay may be integrated into the electronic brake control module rather than being a conventional externally replaceable relay.
The exact diagnostic procedure varies by model, year, ABS configuration and hydraulic control module.
C0120 vs C0110
C0110 and C0120 can both involve the ABS pump motor system, but they should not be treated as identical codes.
C0110 is commonly associated with an ABS pump motor circuit malfunction, while C0120 on applicable GM systems specifically identifies an open pump motor relay circuit.
Related ABS Codes
- C0110 – Pump Motor Circuit Malfunction.
- C0111 – Pump Motor Circuit Range/Performance.
- C0113 – Pump Motor Circuit High.
- C0114 – Pump Motor Circuit Open.
- C0115 – Pump Motor Performance.
- C0121 – Valve Relay Circuit Fault on applicable vehicles.
- C0020 – ABS Pump Motor Control on applicable systems.
FAQ About C0120
What does C0120 mean?
C0120 is a chassis/ABS-related diagnostic code. On applicable GM vehicles, it means Pump Motor Relay Circuit Open. The exact meaning can vary by manufacturer and vehicle.
Is C0120 an ABS code?
Yes. C0120 is associated with the chassis and ABS/brake control system on applicable vehicles.
What causes C0120?
Possible causes include an open relay circuit, blown fuse, damaged wiring, poor connector contact, poor ground, relay failure, ABS pump motor problems, or an internal ABS/EBCM fault.
Can a bad ABS pump cause C0120?
It can contribute to ABS pump-related faults, but C0120 should not automatically be interpreted as proof that the pump motor has failed. The electrical circuit and control system should be tested first.
Can I drive with C0120?
Driving should be minimized until the ABS fault is diagnosed. The conventional braking system may still operate, but ABS and related electronic safety systems may be unavailable.
Does C0120 mean the ABS module is bad?
Not necessarily. Wiring, fuses, grounds, connectors, relay circuits and the pump motor should be checked before condemning the ABS control module.
What should I check first for C0120?
Start with the ABS module codes, battery voltage, ABS fuses, power supply, grounds, connectors and the pump motor relay circuit. Then follow the manufacturer's diagnostic procedure.
Final Result
C0120 is an ABS/chassis-related diagnostic trouble code whose exact definition depends on the vehicle manufacturer. On applicable GM vehicles, it means Pump Motor Relay Circuit Open.
The correct diagnosis should include the ABS fuse, power supply, grounds, relay/control circuit, wiring, connectors, ABS pump motor and EBCM before replacing expensive components.
Always use the vehicle-specific wiring diagram and manufacturer service information for connector pinouts, voltage specifications, relay operation, pump testing and final repair procedures.