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The diagnostic trouble code A118 in the CAS (Car Access System) module of your vehicle indicates an implausible road speed signal. This code suggests that the road speed signal received by the CAS module is not within the expected range or is inconsistent with other sensor inputs.

Here are some possible causes and solutions for this issue:

1. Wheel speed sensor malfunction: Check the wheel speed sensors to ensure they are functioning properly. These sensors provide the road speed information to various systems, including the CAS module. Inspect the sensors for any signs of damage, loose connections, or debris. Replace any faulty sensors and clean the sensor area if necessary.
2. Faulty wiring or connections: Inspect the wiring harness and connectors associated with the wheel speed sensors and CAS module. Look for any signs of damage, loose connections, or corrosion. Repair or replace any damaged wiring or connectors as needed. Faulty wiring or connections can cause erratic or inconsistent road speed signals.
3. ABS (Anti-lock Braking System) malfunction: If your vehicle is equipped with ABS, a malfunction in the ABS system can affect the road speed signal sent to the CAS module. Have the ABS system checked by a professional mechanic using a diagnostic scan tool. If any fault codes are present in the ABS system, address them accordingly.
4. CAS module malfunction: In rare cases, the CAS module itself may be faulty and not properly interpreting the road speed signal. A professional diagnostic scan and inspection of the CAS system are recommended to identify and resolve any component failures.

It’s important to note that these suggestions are general, and the specific diagnostic and repair procedures may vary depending on the make, model, and year of your vehicle. If you are unsure or uncomfortable performing these steps yourself, it is recommended to seek assistance from a qualified mechanic or dealership. They will have the expertise and tools to accurately diagnose and resolve the road speed signal implausibility issue indicated by the A118 CAS diagnostic trouble code.