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The switch telling the vehicle which range the selector is in, and the reason it matters beyond gear selection is that it is a permission device: it decides whether the engine may crank at all, when reverse lamps light, and on many vehicles whether the module will allow a shift strategy to run.

Its failures are more often positional than electrical. The switch reads the selector shaft's position, so a linkage out of adjustment, a worn detent or a switch that has rotated slightly on its mounting will report a range that is genuinely wrong while the switch itself is perfectly healthy. A vehicle that starts in neutral but not in park is the classic presentation, and it is an adjustment rather than a part.

The overlap between positions is where the codes come from. These switches use several contacts and the module checks them against each other, so a worn contact bridging two ranges, or a gap where neither is reported, produces circuit and rationality codes without any loss of driveability. Watch the reported range through the whole travel of the selector — the fault shows as a position that flickers, sticks or reads two ranges at once, none of which a static resistance check will reveal.