P1780

Code

GMC P — Powertrain

Park Neutral Position (PNP) Switch Circuit Engine Torque Signal Circuit

Brand: GMC

Type: P — Powertrain

Views: UK: 24 EN: 47 RU: 38

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Page language: EN

Causes

Open or shorted wiring between PNP switch and PCM/TCM
Corroded, loose or damaged PNP switch connector
Failed PNP (park/neutral) switch
Poor ground or reference voltage to switch or control module
Intermittent harness damage (pinched, rubbed through insulation)
Faulty PCM/TCM or software/firmware issue

Symptoms

Malfunction Indicator Lamp (MIL) illuminated and DTC P1780 stored
Vehicle may fail to start in Park/Neutral or start-restrict behavior
Transmission may default to limp/limited shift strategy
Cruise control or torque management features suspended
Intermittent starting or no-start complaints tied to shifter position

What to check

Read and record freeze frame and related transmission codes with a scan tool
Verify transmission range (gear) reported by scan tool matches actual shifter position
Visually inspect PNP switch connector and wiring for corrosion, damage, or loose terminals
Backprobe PNP switch connector with key ON, engine OFF; observe voltage/logic state in Park and Neutral
Perform continuity and resistance checks between PNP connector and PCM/TCM pins; check for shorts to ground or battery
Check power/ground/reference voltages at PCM/TCM and at PNP connector

Signal parameters

PNP/Range switch signals are typically logic-level circuits (0–5 V) or switched battery/reference lines — expected: clear state change when moving to/from Park or Neutral
Resistance across switch contacts when actuated: near 0 Ω (closed); when released: open or >10 kΩ (varies by design)
Reference voltage at module pins: approx. 5 V or battery voltage depending on vehicle design — verify against factory data
Module-reported gear position should change to PARK or NEUTRAL when shifter is in those positions

Diagnostic algorithm

Connect a capable scan tool. Read and record P1780 and any related codes; note freeze frame data and reported gear position.
Confirm symptom: with ignition ON (engine OFF) put shifter in PARK and NEUTRAL and verify scan tool shows corresponding positions.
Visually inspect PNP switch, connector, and wiring at the transmission/shifter for corrosion, bent pins, damage, or fluid contamination; repair obvious issues.
Backprobe the PNP switch connector with key ON, engine OFF. Observe voltage/state in Park and Neutral. Compare to expected logic changes and to factory specs.
With connector disconnected, check continuity between PNP pins and the PCM/TCM pins. Also check for short to battery and short to ground.
If wiring is intact, bench test or actuate the PNP switch and verify contact resistance and switching action. Replace switch if it fails to switch cleanly or shows high resistance.
Repair or replace damaged wiring, connectors, or switch as found. Restore harness routing and secure to prevent future chafe.
After repairs, clear codes, perform a key-on/engine-run verification and a road test to confirm the code does not return.
If wiring and switch check good but code persists, consider module diagnostics/programming or replacement after verifying inputs and grounds to the module.

Likely causes

Wiring harness chafe at shift lever or trans top cover
Contaminated or corroded PNP connector at transmission
Broken solder joint or terminal in PNP switch
Failed PNP switch due to mechanical wear

Fault status

⚠️ Status

DTC P1780 stored — Park/Neutral Position switch circuit (engine torque signal) fault. MIL illuminated; transmission may enter limp mode and starting/torque management may be restricted.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-2.5 hours

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P1780 — Park Neutral Position (PNP) Switch Circuit Engine Torque Signal Circuit