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C0121 C0xxx · 底盘（C） 高严重度
Brake Fluid Pressure / Differential Pressure Switch Fault
The pressure differential switch indicates an imbalance between brake circuits — a genuine hydraulic warning, not an electronics nuisance.
严重程度说明： 高——立即停车诊断，继续行驶有扩大损坏的风险
你会注意到的故障现象
- Brake warning lamp on
- Low or spongy pedal
- Visible fluid loss
常见原因
按可能性从高到低排列——从最上面开始查。
- 1 Brake fluid leak in one circuit
- 2 Air in the system
- 3 Failed pressure differential switch
- 4 Worn master cylinder seals
如何排查
先做花钱最少、侵入最小的检查——大多数这类码不是零件问题。
- ✓ Check fluid level and look for leaks at each wheel and along the lines.
- ✓ Inspect the master cylinder for external leakage.
- ✓ Bleed the system if air is suspected.
- ✓ Test the switch only after confirming hydraulics are sound.
维修方案
- Repair the leak and bleed the brakes
- Replace the master cylinder
- Replace a faulty switch
C0121 —— 常见问题
Is it safe to drive? +
Not until you have confirmed there is no leak. A hydraulic imbalance means degraded braking.
Can low fluid alone trigger it? +
Yes — a low reservoir is the most common cause. Top up, then find where the fluid went.