They are inexpensive parts that fail often, usually by cracking at the spline or pulling off the shaft.
A knob is replaced when it has split, when the printed symbol has worn away, or when the retaining clip has broken and the knob falls off in use. Where the knob operates a folding or walk-in mechanism, a missing knob means the mechanism cannot be released at all, which makes it a functional part rather than a cosmetic one.
Match the knob by shape and by shaft type. Two knobs that look alike can have a different number of splines or a different depth of socket, and the driver and passenger sides frequently differ. The listing photographs show the shaft and the marking, which is the reliable way to compare against the part in your hand.
