What Does P1591 Mean?
This code indicates the neutral position switch circuit is reading an abnormally low voltage. The signal is being pulled to ground, which could be caused by a short to ground in the wiring, an internally shorted switch, or a failed pull-up resistor. The PCM may not be able to determine the correct gear selection, affecting shift strategy and starting logic.
Common Causes
30%
Short to ground in the neutral position switch signal wiring
30%
Internally shorted neutral position switch
20%
Water intrusion into the switch or connector causing a ground path
15%
Chafed wiring contacting the transmission case or frame
5%
Contaminated switch contacts from transmission fluid leak
Diagnostic Steps
1
Monitor the neutral position switch signal voltage on live data; verify it reads near 0V in all positions, confirming the low-input condition.
2
Disconnect the neutral position switch connector and recheck the signal voltage at the PCM side—if it goes high, the switch is internally shorted.
3
If voltage remains low with the switch disconnected, the wiring has a short to ground; inspect the harness from the PCM to the switch.
4
Check for transmission fluid leaking into the switch connector, which can create conductive paths.
5
Replace the neutral position switch if internal short is confirmed; repair wiring if a harness fault is found.
Estimated Repair Cost
$100 - $350
Parts + labor, varies by vehicle and location
This code indicates the neutral position switch circuit is reading an abnormally low voltage. The signal is being pulled to ground, which could be caused by a short to ground in the wiring, an internally shorted switch, or a failed pull-up resistor. The PCM may not be able to determine the correct g...
The most common cause of P1591 (Neutral position switch circuit low input) is: Short to ground in the neutral position switch signal wiring
Typical repair costs for P1591 range from $100 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
The vehicle may start in gear if the PCM cannot correctly identify neutral/park position. Transmission shift quality and strategy may be compromised, leading to harsh or inappropriate shifts.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1591 to identify the root cause.
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Quick Info
Category
Powertrain
System
Transmission / Starting System
Difficulty
Type
Manufacturer
Recommended Tools
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