C1023 — Rear left wheel speed sensor circuit high

C1023 is stored when the ECU sees signal too high on ABS / wheel speed (rear left). That describes how the circuit is behaving, not which part has failed — the wiring, the connector and the component itself can all produce it, which is why the checks below start with the cheapest.

Common causes

The cause is almost always one of a few things. Here they are — C1023 rarely comes from anything outside this list:

• Open ground for the sensor, letting the signal float up
• Signal line shorted to a supply voltage
• Sensor failed open, which the module reads as maximum
• Connector fault interrupting the return path

Where to start

In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:

1. Read the live value and compare with what the conditions should produce
2. Check the sensor ground continuity back to the module
3. Unplug the sensor: if the reading changes, the wiring is intact and the sensor is suspect
4. Inspect the connector for corrosion or a spread terminal
5. Replace the sensor once ground and signal wiring are confirmed good

That is what holds for C1023 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.