Faulty gear range sensor or transmission range switch; Damaged or corroded wiring in the gear indicator circuit; Poor electrical connection at the transmission or PCM; Failed instrument cluster or body control module (BCM) affecting the display; Internal PCM fault causing false detection; Aftermarket accessories or poor grounds interfering with the circuit

DTC codes / GM codes / P1520

P1520 · GMGear Indicator System

Q: What does code P1520 mean?

Gear Indicator System. P1520 is a GM-specific code indicating the powertrain control module (PCM) has detected a fault in the gear indicator system, which communicates the selected gear range to the driver and other modules. The PCM monitors the circuit that controls the gear indicator display and sets this code when the signal is out of expected range or missing.

P1520 is a GM-specific code indicating the powertrain control module (PCM) has detected a fault in the gear indicator system, which communicates the selected gear range to the driver and other modules. The PCM monitors the circuit that controls the gear indicator display and sets this code when the signal is out of expected range or missing.

SystemPowertrain

Code typeGM specific

SeverityMedium

Fix difficultyIntermediate

AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

The gear indicator on the instrument cluster may be blank, flashing, or show the wrong gear
The check engine light or service vehicle soon light is illuminated
The transmission may shift erratically or feel like it is in the wrong gear
Some drivers notice no symptoms besides the warning light
The vehicle may enter limp mode or limit engine power
Other dashboard displays or warning chimes may behave oddly

Common causes, most likely first

Faulty gear range sensor or transmission range switch
Damaged or corroded wiring in the gear indicator circuit
Poor electrical connection at the transmission or PCM
Failed instrument cluster or body control module (BCM) affecting the display
Internal PCM fault causing false detection
Aftermarket accessories or poor grounds interfering with the circuit

How to diagnose and fix it

Use a scan tool to read all stored codes and freeze frame data to confirm P1520 is current
Inspect the transmission range switch and its connector for corrosion, looseness, or damage
Check the wiring harness from the transmission to the PCM for chafing, breaks, or shorts
Test the range switch with a multimeter for proper continuity in each gear position
Verify power and ground at the instrument cluster and BCM if the display is erratic
Repair or replace faulty components as needed, then clear codes and test drive
If the code persists, perform a pinpoint test for the gear indicator circuit using factory service information

Don't overlook the range switch

A dirty or misadjusted transmission range switch is a common cause that is often overlooked because it does not always trigger other transmission codes.
The gear indicator system shares wiring with the reverse lights and backup camera on many GM models, so those circuits can be affected too.
A weak battery or poor ground can cause the PCM to misread the gear indicator signal, leading to a false P1520.
Aftermarket remote starters or alarm systems tapped into the gear position circuit can interfere with the signal and set this code.
The code can be set by a faulty instrument cluster even when the transmission range switch is working perfectly, so do not replace the switch without testing.

Can I keep driving with P1520?

You can usually drive with P1520, but the transmission may not shift correctly or the gear indicator may be inaccurate, so avoid aggressive driving and have it diagnosed soon to prevent further transmission damage.

Tools that help

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