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A hub bearing that has developed play or roughness — and a cause that reaches further than most people expect, because on many vehicles the wheel speed sensor is built into the bearing.

That is why it belongs on a list of electrical-sounding faults. Play at the hub changes the gap between sensor and tone ring as the wheel turns, so the signal weakens and strengthens once per revolution. The result reads exactly like a failing sensor, and replacing the sensor changes nothing.

The magnetic encoder ring is usually part of the bearing seal too, so a bearing fitted the wrong way round, or one whose encoder was scratched or wiped during fitting, produces a wheel speed fault on a brand-new part.

The mechanical symptoms are the giveaway when they are present: a droning or rumbling that changes with speed, and often changes as the car is steered — loading the bearing by cornering makes it louder or quieter. Rocking the wheel at twelve and six o'clock with the car raised finds obvious play.

But it can produce the electrical fault with no noise at all, which is the case worth remembering.