P1787 Transmission Range Sensor Circuit Malfunction
The P1787 code indicates that the Powertrain Control Module (PCM) has detected a voltage signal from the Transmission Range Sensor (TRS) / Park-Neutral Position (PNP) switch circuit that is outside the calibrated range during a self-test (KOEO or KOER). This means the PCM cannot reliably determine the gear selector position (Park, Reverse, Neutral, Drive), affecting starting strategy, shift scheduling, and torque control.
01. Most Common Causes
- Faulty Transmission Range Sensor (TRS) / PNP switch
- Open, short-to-ground, or short-to-voltage in the TRS circuit wiring or connectors
- Misadjusted or damaged shift linkage or transmission cable
- Faulty PCM (rare)
02. Symptoms You Will Notice
- Engine may not start or starts only in certain gear positions
- Harsh or erratic shifting, or transmission stuck in limp mode
- Incorrect gear indicator display on the dashboard
- Check Engine Light illuminated
03. Step-by-Step Diagnosis
Scan for codes and record freeze frame data
Inspect TRS/PNP switch connector and wiring for damage, corrosion, or loose pins
Test TRS/PNP switch resistance and continuity per manufacturer specs
Check shift linkage adjustment and mechanical operation
If all else passes, test PCM input/output signals and consider PCM replacement
1. Frequently Asked Questions
Is it safe to drive?▼
It is moderately safe to drive short distances, but the transmission may not shift properly or the engine may not start in Park or Neutral. Avoid prolonged driving until repaired.
How much does it cost to fix?▼
Repair costs typically range from $150 to $400 for switch replacement, plus $50–$150 for diagnosis. Wiring repairs or PCM replacement can increase costs significantly.