Cause
Faulty park neutral switch
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.
Typically a moderate job.
Codes this can cause (5)
Codes where this is a likely explanation first, rather than merely a possible one.
- P084F Park/Neutral Switch Output Circuit Likely for this code Moderate
- P0850 Park/Neutral Switch Input Circuit Likely for this code Moderate
- P0851 Park/Neutral Switch Input Circuit Low Likely for this code Moderate
- P0852 Park/Neutral Switch Input Circuit High Likely for this code Moderate
- P0853 Drive Switch Input Circuit Moderate
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