C0666 code meaning
Right Rear Wheel Speed Sensor B Circuit/Open
What C0666 means
The second speed channel at the right rear wheel is open: a wire, terminal or the sensor itself no longer conducts. C0666 is not documented by any carmaker. Where the repair ends depends on the hub design. Schaeffler's wheel bearing brochure notes that some third-generation bearings carry all speed sensors inside the bearing unit, so an internal open there can mean replacing the whole hub assembly. The loss also affects the parking brake. A ZF patent on electric parking brakes handles a missing or implausible speed signal by closing the brake only while the switch is held and with a steady rise in force, so the car stays stable; full clamping follows only once maximum force is reached. Normal ABS often stays available through sensor A.
C0666 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of C0666
- Warning for brake or stability system on every start
- Electric parking brake may only apply gradually while the switch is held
Common causes of C0666
- Most likelyHarness or connector open at the right rear wheel
- PossibleSensor inside the hub bearing unit failed open
- Less likelyInput stage for channel B in the brake module defective
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Rr wss bSensor wiring
What each part of C0666 means
| C | Chassis system: brakes, ABS, steering and suspension. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 66 | The exact fault in that area. |
What to do next
- Note C0666 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about C0666
What does C0666 mean?
C0666 means "Right Rear Wheel Speed Sensor B Circuit/Open". The second speed channel at the right rear wheel is open: a wire, terminal or the sensor itself no longer conducts. It is a generic code, so it means the same on every make.
Is it safe to drive with C0666?
Drive gently and only to a workshop. With C0666 the car can go into limp mode (reduced power) to protect itself.
Will C0666 fail an emissions test?
C0666 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is C0666 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2.5 hours of work. This is a job for a workshop.
Will C0666 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.