Here's why:
* The NSS's Job: This switch prevents the car from starting unless it's in Park or Neutral. It also ensures that the car won't start while the transmission is in gear. It's a crucial safety feature. In your case, it's working fine for starting, but it's failing to allow the car to stay running when the transmission is engaged.
* Failure Modes: The NSS can fail in several ways. It might be:
* Faulty Switch: The internal contacts are worn, corroded, or broken, preventing proper electrical connection when the shifter is in Drive or Reverse.
* Faulty Wiring: The wiring harness to the switch could be damaged, frayed, or corroded.
* Shift Linkage Problem: The linkage connecting the shifter to the transmission might be out of adjustment, preventing the switch from engaging properly.
Troubleshooting Steps (in order of increasing complexity):
1. Check the Shift Linkage: Carefully inspect the linkage connecting the shifter to the transmission. Make sure it's not bent, broken, or loose. A slight adjustment might be needed.
2. Inspect the NSS (Neutral Safety Switch): Locate the NSS (usually on the transmission itself, near the shifter linkage). Visually inspect it for any obvious damage. You might need a diagram or repair manual to identify it.
3. Test the NSS (Advanced): You'll need a multimeter to test the continuity of the NSS in different gear positions. A repair manual will provide specific instructions on how to perform this test. This step often requires some mechanical knowledge and comfort working under the dash and around the transmission.
4. Check the Wiring: Visually inspect the wiring harness connected to the NSS for any damage, corrosion, or loose connections.
If you are not comfortable with any of these steps, especially electrical work or working under the car, take it to a mechanic. Improperly diagnosing or repairing this issue could lead to further damage.
Other (Less Likely) Possibilities:
* Transmission Problems: While less likely given the symptoms, a serious transmission problem *could* also cause this. However, this usually involves other symptoms, like slipping, hard shifting, or unusual noises.
* Ignition System Issues: Although less likely, a problem in the ignition system could intermittently cut power when under load (driving or reversing).
The neutral safety switch is the most probable reason, but a professional diagnosis might be needed to rule out other issues.
