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

Causes

• Failed or intermittent PNP (park/neutral) switch
• Open, shorted or damaged wiring between PNP switch and PCM/TCM
• Corroded/loose connector or poor pin contact at the PNP switch or PCM/TCM
• Poor or missing ground or 5V reference to the switch
• Faulty PCM/TCM or internal module input fault
• Incorrect repair/installation or mechanical misadjustment of the PNP switch

Symptoms

• MIL/Check Engine lamp illuminated
• Transmission may refuse to shift out of Park or may default to limp mode
• Starter cranks but engine may not start (if PNP affects starter inhibit)
• Inconsistent gear selection indication or incorrect PRNDL display
• Stalling or torque management events during gear selection (rare)

What to check

• Retrieve DTC(s) and freeze frame data with a scan tool; note conditions when code set
• Verify vehicle battery voltage is within normal range
• Visually inspect PNP switch, connectors and harness for damage, corrosion or pin loss
• Backprobe PNP switch connector and measure signal/voltage while moving shifter through positions
• Check for proper 5V reference and ground at the switch connector (if applicable)
• Measure continuity/resistance of PNP switch circuits (should show closed in Park/Neutral, open in other positions)

Signal parameters

• PNP switch typically provides discrete Park and Neutral circuits to the PCM/TCM (not a variable sensor)
• Expected behavior: corresponding circuit shows continuity (low ohms) in Park or Neutral and open in other gear positions
• Voltage behavior (vehicle dependent): signal may be near 0 V (grounded) when closed or pulled to ~5 V when open — confirm exact wiring in vehicle service data
• Reference voltage to switch commonly ~5 V; ground should be vehicle chassis ground — verify present before replacing parts
• Noisy, fluctuating or intermittent signals indicate wiring/connector faults or failing switch

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