C004B – Brake Fluid Level Sensor/Switch Circuit Open (OBD2)
C004B means "Brake Fluid Level Sensor/Switch Circuit Open". The control unit sees an open circuit: the electrical connection to the brake fluid level and pad wear sensor is interrupted, so no current can flow when the component is commanded or read.
Last updated
C004B quick summary
| Fault code | C004B |
|---|---|
| Meaning | Brake Fluid Level Sensor/Switch Circuit Open |
| System | Chassis |
| Vehicles | Generic OBD2 |
| Severity | High |
| Can I drive? | Check immediately. If the brake fluid is low and the pedal is soft or goes further down than usual, do not drive. A pad wear warning allows careful driving to a workshop, but replace the pads soon to avoid damaging the discs. |
| Repair cost | €10 – €400 |
How brake fluid level, pad wear and brake temperature are monitored
These codes mean the brake fluid level is low, a brake pad wear sensor has been triggered or interrupted, the fluid level switch circuit is faulty, or the calculated brake temperature is too high.
A float switch in the brake fluid reservoir warns when the fluid level drops. Because the level falls slowly as the pads wear, a low level often means worn pads – or a leak. Many cars also have pad wear sensors: a wire loop embedded in the pad that is ground through when the pad is nearly worn, or interrupted when it has been cut through.
The ABS/ESP module also estimates the brake temperature from how often and how hard the brakes are used. If it calculates overheating (for example on long descents), it stores a code and may limit some functions.
Brake system warnings must always be taken seriously: a low brake fluid level combined with a soft pedal can mean a leak in the hydraulic system.
C004B is a chassis code in the standard (SAE) numbering. Some manufacturers – for example GM, Opel and Ford on certain models – use their own meanings for some C codes, so always check the text your scan tool shows for your vehicle.
Symptoms of C004B
- Red brake warning light or brake fluid message
- Brake pad wear warning
- Soft or long brake pedal (leak)
- Squealing or grinding from the brakes (worn pads)
- Brake fade or burning smell after long descents (brake temperature)
Most common causes of C004B (most likely first)
Mechanic's Pro Tip
Always fit a new pad wear sensor when replacing pads on cars that use them – a sensor that has been ground through cannot be reused, and an old cable can trigger a false warning.
1.Unplugged, loose or corroded connector
Most likelyOften after previous repair work in the area.
2.Broken wire in the harness
Typical at bends, near the engine mounts and where the harness rubs on brackets.
3.Burnt-out winding or internal break in the brake fluid level and pad wear sensor
4.Worn brake pads
Triggers the wear sensor and lowers the fluid level in the reservoir.
5.Brake fluid leak
At a caliper, brake line, hose or master cylinder.
6.Damaged pad wear sensor or wiring
Wear sensors are often damaged during pad replacement or by heat.
7.Overheated brakes
Long descents, dragging calipers or a stuck handbrake.
Step-by-step diagnostic guide
- 1
Check fluid level and pedal
Check the brake fluid level and the pedal feel with the engine running. A soft pedal with low fluid means a leak – do not drive.
- 2
Measure resistance and continuity
Unplug the brake fluid level and pad wear sensor and measure its resistance across the terminals. An infinite (OL) reading confirms an internal break. If the component is fine, check each wire back to the control unit for continuity.
Multimeter: Solenoids and heaters typically read a few ohms to a few dozen ohms; "OL" means open. Wire continuity: below about 1 Ω.
- 3
Check pad thickness
Inspect all brake pads and discs. Low fluid with worn pads is normal; topping up without changing pads only hides the warning.
Multimeter: Most pads should be replaced at about 2–3 mm of friction material.
- 4
Look for leaks
Check calipers, hoses, lines and the master cylinder for wetness.
- 5
Check sensor and switch circuits
Check the wear sensor loop and the level switch with a multimeter and inspect the connectors.
- 6
Repair and reset
Replace pads and wear sensors, repair leaks, bleed the brakes, then clear the codes and reset any service indicator.
Frequently asked questions
Can I keep driving with C004B?
Check immediately. If the brake fluid is low and the pedal is soft or goes further down than usual, do not drive. A pad wear warning allows careful driving to a workshop, but replace the pads soon to avoid damaging the discs.
How much does it cost to fix C004B?
Typical repair costs are 10 € – 400 €, depending on the cause and the vehicle. The most common fixes are: unplugged, loose or corroded connector; broken wire in the harness; burnt-out winding or internal break in the brake fluid level and pad wear sensor. Always confirm the cause with the tests above before buying parts.
Should I top up the brake fluid?
Only after checking why it is low. If the pads are worn, the level rises again when new pads are fitted. If there is a leak, topping up is not a repair.
Is a pad wear warning urgent?
It means the pads are close to the minimum. Replace them soon – driving on worn-out pads damages the discs and reduces braking performance.
What is the difference between C004B and similar codes?
C004B belongs to a group of codes for the same component that differ in the type or location of the fault: C0049 – Brake Fluid Level; C004A – Brake Lining Wear Sensor; C0072 – Brake Temperature Too High. The tests on this page apply to the whole group; the type of fault tells you where to start.
Related fault codes
This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.