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A caliper not releasing fully, so the pad remains in contact with the disc. The drag is small enough not to be obvious and large enough to matter: it generates heat continuously, wears that pad and disc rapidly, and costs measurable fuel.

Its effect on the diagnostic systems is through wheel speed. A dragging corner turns marginally slower than the others, and the stability and traction systems compare speeds continuously — so a persistent small difference reads as slip and produces codes with no fault in any sensor. A wheel noticeably hotter than its opposite after a run is the confirming check and costs nothing.

Seized slider pins and corroded piston seals are the usual causes, along with a collapsed flexible hose acting as a one-way valve — that last one is worth knowing, since it lets pressure in and not out, so the caliper drags while the caliper itself is perfect. Check that the wheel spins freely with the vehicle raised and compare against the other side.