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DTC codes / Mazda codes / P1516

P1516 · MazdaGear Change Neutral Drive Fault

P1516 on Mazda vehicles indicates that the powertrain control module (PCM) has detected an implausible signal from the neutral switch or gear position sensor circuit during a gear change, specifically when the transmission is expected to be in neutral. The PCM compares the expected neutral status with the actual voltage from the switch and sets the code when the two disagree for a calibrated period.

SystemPowertrain
Code typeMazda specific
SeverityMedium
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • Illumination of the check engine light or malfunction indicator lamp (MIL) on the dashboard.
  • Often no symptoms besides the warning light, especially if the fault is intermittent.
  • Rough or delayed engagement when shifting from neutral to drive or reverse.
  • Unexpected engine idle fluctuations when the transmission is in neutral.
  • In rare cases, the transmission may default to a limp mode with harsh shifting.
  • Difficulty starting the engine if the PCM incorrectly believes the car is in gear.

Common causes, most likely first

  1. Faulty neutral safety switch or gear position sensor on the transmission.
  2. Damaged or corroded wiring harness connector at the switch.
  3. Open or short circuit in the signal wire between the switch and the PCM.
  4. Poor ground connection for the switch or PCM.
  5. Internal PCM software glitch or faulty PCM (least likely).
  6. Mechanical wear in the shift linkage causing the switch to misalign.

How to diagnose and fix it

  1. Connect a scan tool and verify P1516 is the only code; check freeze frame data for conditions when it set.
  2. Inspect the neutral switch connector and wiring for corrosion, chafing, or loose pins.
  3. Test the switch with a multimeter for continuity in neutral and open circuit in gear, following Mazda service information.
  4. Check the switch adjustment and shift linkage for proper alignment; adjust or replace as needed.
  5. Perform a voltage drop test on the ground circuit to ensure less than 0.1 volts.
  6. If wiring and switch test good, substitute a known-good PCM and retest before replacement.
  7. Clear codes and perform a road test with several gear changes to confirm the repair.

Neutral switch faults that mimic transmission failure

  • A misadjusted neutral switch can cause P1516 without any driveability complaint, so always check adjustment before condemning parts.
  • Corrosion inside the connector is a common cause on older Mazdas, even when the switch itself tests fine.
  • The code can set intermittently due to vibration, so wiggle-test the harness while monitoring switch status on a scan tool.
  • Replacing the PCM without verifying the switch circuit is an expensive misdiagnosis trap.
  • Some Mazda models use a combined neutral and backup light switch; a failure in the backup light circuit can affect neutral signal.

Can I keep driving with P1516?

The vehicle is generally safe to drive with P1516, but avoid aggressive gear changes and have the fault diagnosed soon, as a misreported neutral signal can affect starting and idle quality.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Mazda vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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