Code
P1780
SATURN
P — Powertrain
Park Neutral Position PNP Switch Circuit Engine Torque Signal Circuit
AI status
Completed
Completed
100%
Causes
- Faulty Park/Neutral Position (PNP) switch / transmission range switch
- Damaged, chafed or disconnected wiring harness between PNP switch and PCM
- Corroded or loose connector pins at switch or PCM
- Short to battery or short to ground in the PNP circuit
- Poor PCM ground or power supply problem
- Intermittent internal PCM input fault
Symptoms
- MIL/Check Engine lamp illuminated
- Vehicle may not crank or start in Park/Neutral as expected
- Starter engage problems (no-crank in Park unless shifted to Neutral or vice versa)
- Transmission may go into limp/backup mode or have odd shift behavior
- Intermittent starting or erratic torque management
What to check
- Scan for additional codes and freeze frame data; note when code set and vehicle conditions
- Visually inspect PNP switch, wiring harness and connectors for damage, corrosion, pin push-out or water intrusion
- Backprobe PNP switch connector with ignition ON and shift lever in Park and Neutral; observe signal voltages
- Check continuity/resistance between PNP switch and PCM pin with connector disconnected
- Check for shorts to battery or ground in PNP wiring
- Verify PCM ground(s) and power supply voltages are within specification
Signal parameters
- PNP switch is a discrete on/off or range sensor; expected behavior: a clear switched signal that changes with selector position (Park/Neutral vs other gears)
- Typical voltages: near battery voltage or reference voltage when switch closed, near 0V or open when switch open (exact values vary by model — check vehicle data)
- Continuity: closed switch should show low resistance across the intended terminals in Park/Neutral; open in other positions
- If PCM expects a pulsed torque signal derived from PNP, the waveform should be a stable square/steady state corresponding to switch position
Diagnostic algorithm
- Read and record all diagnostic trouble codes and freeze frame data. Note selector position when code set.
- Perform a visual inspection of the PNP switch, connector, and harness along the shift linkage and transmission. Repair obvious damage.
- With ignition ON (engine OFF), backprobe the PNP connector and observe voltage with the gear selector in Park and in Neutral. Compare to expected behavior (switch changes state).
- Check continuity/resistance between the PNP switch terminals and the PCM connector pins. Repair any open or high-resistance circuits.
- Inspect for shorts to battery or ground. Disconnect the PNP connector and check for unexpected voltage on circuit wires.
- Wiggle test harness and connectors while monitoring signal to find intermittent faults. Clean/repair connectors as needed.
- If wiring and connector check good, replace the PNP switch and re-test.
- If problem persists after switch replacement, verify PCM power/ground integrity and consider PCM input circuit testing or replacement per factory procedures.
- Clear codes and perform functional test/road test to confirm repair.
Likely causes
- Connector contamination or pin corrosion at the PNP switch
- Broken or worn wire where harness flexes (shift linkage area)
- PNP switch has failed electrically (open/short or intermittent contacts)
- Aftermarket modifications or repairs disturbed wiring
- PCM input circuit intermittent or damaged
Fault status
Status
PCM detected invalid or missing Park/Neutral Position (PNP) switch signal used for engine torque control; circuit malfunction stored.
Repair difficulty: Medium
Diagnostic time: 0.5-1.5 hours
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