Brake Switch Circuit Malfunction
This code indicates an issue with the brake switch circuit.
Heads up: this code wasn't in our hand-written library yet, so DiagCoach AI built this page on the fly. Double-check critical specs against service info before condemning parts.
What it means (plain English)
The brake switch tells the computer when you press the brake pedal. If this switch is malfunctioning, the computer won't know when you're trying to stop, which can lead to problems with braking systems and safety features.
What the computer is actually seeing
The computer monitors the brake switch input, which should show a change in voltage (typically from 0 volts to 12 volts) when the brake pedal is pressed.
What a healthy reading looks like
Voltage should be around 0V when brake is off and close to 12V when brake is pressed.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Check the brake fluid level. If low, top off and see if the code returns.
- 22. Inspect the wiring and connectors at the brake switch for damage or corrosion. Repair as necessary.
- 33. Use a multimeter to check voltage at the brake switch. Expect to see 0V when the pedal is not pressed and near 12V when pressed. If not, the switch may be faulty.
- 44. If the switch appears fine, check wiring continuity from the switch to the PCM. Look for shorts or opens in the circuit.
- 55. If all tests pass, consider testing or replacing the PCM.
Common causes
- Faulty brake switch
- Wiring issues in the brake switch circuit
- Poor connector condition
- Faulty PCM (Powertrain Control Module)
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with code C0120?
It's not recommended to drive as the brake system might not function properly, impacting safety.
Will this code come back after clearing?
Yes, if the underlying issue isn't fixed, the code will return.
Why does this code pair with C0121?
C0121 often indicates a related issue in the same brake switch circuit, further confirming there's a problem.
Working a real vehicle right now?
Let DiagCoach walk you through it live with your specific symptoms, vehicle, and what you've already checked.
Start guided diagnostic →