Summary
C0300 indicates that the ABS module has detected a malfunction in a rear wheel speed sensor or its circuit. Unlike codes C0045 and C0050 that specify left or right, C0300 is a general rear speed sensor code used by some manufacturers when the module detects a problem but does not differentiate between the two rear sensors — or on vehicles with a single rear axle speed sensor. The most common cause is a failed sensor (35%), followed by wiring damage (25%) and tone ring issues (20%). ABS and stability control are disabled, but conventional brakes work normally.
Severity: High — ABS and stability control are affected
Safe to drive: Yes, with caution — normal brakes work but ABS and stability systems are compromised
Repair cost: $50–$350 depending on cause
DIY difficulty: Easy to Moderate
What does C0300 mean?
Some vehicles — particularly trucks and SUVs with solid rear axles — use a single speed sensor mounted on the rear differential or transfer case output rather than individual sensors at each rear wheel. This sensor reads a reluctor ring on the ring gear or output shaft and provides a combined rear-axle speed signal to the ABS module. Other vehicles that use individual rear sensors may still set C0300 as a general "rear speed" fault.
The ABS module monitors this rear speed signal and compares it with front wheel speeds. C0300 is set when the rear speed signal is absent, erratic, or does not match expected values based on the front wheel speeds and vehicle operating conditions. On vehicles with a differential-mounted sensor, the code cannot distinguish between a left or right rear problem — it only knows the combined rear speed is faulty.
Once set, the ABS module disables anti-lock braking for the rear wheels and may disable traction control and stability control entirely, depending on the vehicle. The ABS and traction control warning lights illuminate on the dashboard.