* Faulty Park Neutral Safety Switch (PNSS): This switch tells the car's computer the transmission's position. If it's malfunctioning, it might be sending the wrong signal, causing the headlights (and possibly other accessories) to shut off. This is a common culprit for this type of problem.
* Wiring Issue: A short circuit or damaged wire in the headlight circuit or related to the PNSS could interrupt the power supply to the headlights when the transmission shifts to park. This could be anywhere from the switch itself to the headlight fuse or even a corroded connection.
* Body Control Module (BCM) Problem: The BCM controls many electrical functions in the vehicle. A faulty BCM could be misinterpreting signals or experiencing an internal fault, leading to the headlights turning off.
* Faulty Headlight Switch: Although less likely, a malfunctioning headlight switch could be the issue.
* Fuse or Relay: A blown fuse or a failing relay in the headlight circuit could also cause the problem.
Troubleshooting Steps:
1. Check the fuses: Inspect the fuses related to the headlights and the ignition system in your fuse box (usually under the hood or inside the cabin). Replace any blown fuses.
2. Check the Headlight Switch: Try turning the headlights on and off manually several times to see if the switch is faulty. Sometimes a faulty switch makes intermittent contact.
3. Inspect Wiring: Carefully examine the wiring harness leading to the headlights and the PNSS for any visible damage, such as frayed wires or corrosion.
4. Have the PNSS checked: A mechanic can easily test the PNSS to see if it's functioning correctly. This is often the first thing to check.
5. Diagnostic Scan: Take your Envoy to a mechanic or auto parts store that can perform a diagnostic scan. The scan will read any trouble codes stored in the car's computer, which might pinpoint the exact cause of the problem. This is the most effective way to determine the root cause.
Since this is a safety concern (driving at night without headlights is dangerous), it's best to get this issue diagnosed and repaired by a qualified mechanic as soon as possible. Don't attempt extensive electrical repairs yourself unless you have experience working with automotive electrical systems.
