Volvo's ABS system can be diagnosed without expensive testers until late 1995 from the engine bay diagnostic box. From 1996 onwards, codes are read from the center console OBDII socket, but the operating principle remains the same: flash codes.

On 1988–1995 models: The diagnostic socket (A-box) is located at the base of the driver-side shock absorber strut. Insert the socket wire into hole 3. On 1996–1998 models: Use the center console OBDII socket (requires a DIY reader; see earlier instructions).

Turn on the ignition (position II). Press the diagnostic box button for approximately 1 second. Count the red LED flashes. The code is three digits (e.g., 1-2-1). Press again to read the next code, until the same code repeats.

Code Explanation 1-1-1 No fault codes (System OK). 1-2-1 Left front sensor: Faulty signal below 40 km/h. 1-2-2 Right front sensor: Faulty signal below 40 km/h. 1-2-3 Left rear sensor: Faulty signal (NOTE: Rigid axle has only one sensor). 1-2-4 Right rear sensor: Faulty signal (Multilink models only). 1-2-5 Multiple sensor signals invalid for extended period. 1-3-5 Control unit internal fault. 1-4-1 Brake pedal sensor: Short to ground or voltage. 1-4-2 Brake light switch: Open circuit (break). 1-4-4 Brake discs overheated (based on calculated data). 1-5-1 Left front sensor: Break or short to voltage. 1-5-2 Right front sensor: Break or short to voltage. 1-5-5 Rear axle sensor: Break or short to voltage. 2-1-1 Left front sensor: No signal on acceleration. 2-1-2 Right front sensor: No signal on acceleration. 2-1-5 ABS unit valve relay: Break or short circuit.

If your car has a rigid rear axle (most 740/940 models), above the rear bumper there is only one shared sensor for both rear wheels. It provides speed information to both the speedometer and the ABS system. If the speedometer works but you get a rear ABS code, the fault is often in the wiring between the ABS unit and the instrument cluster.

Dirty sensors: Front sensors accumulate metal dust and dirt, which interferes with the signal. Clean the sensor head and toothed ring (ABS wheel). Broken ABS ring: If code 1-2-1 or 1-2-2 occurs only at low speeds, check if the front hub toothed ring is broken or cracked. Overvoltage protection (Over-relay): If the ABS light is on but you cannot establish connection to diagnostics, check the relay near the ABS unit, which often has a flat fuse on top. If the fuse is blown, the ABS has no power.