* Passlock System Failure: The 2004 Impala likely uses a Passlock system, which is a theft-deterrent system that prevents the car from starting if it doesn't recognize the correct ignition key. A malfunctioning Passlock system can completely disable the car's electrical system except for the security light. This could be due to a bad Passlock sensor in the steering column, a faulty ignition switch, or even a weak battery in the key fob (though usually that just prevents starting, not completely killing power).
* Battery Issues (but likely secondary): While a dead battery is a common cause of power loss, it's less likely to be the *sole* cause if the security light is the only thing working. A completely dead battery would usually result in *no* lights whatsoever. It's more plausible that a very weak battery is exacerbating an underlying Passlock issue.
* Wiring Problems: Damaged or corroded wiring, particularly in the area of the ignition switch or under the dash near the security system components, could interrupt the power flow to the rest of the vehicle. A short circuit could also be the culprit.
* Body Control Module (BCM) Issue: The BCM controls many electrical functions in the car. A faulty BCM could be preventing power from reaching various components, while still allowing the security light to function.
* Security System Module Malfunction: The security system itself could be faulty, causing it to shut down the entire car's electrical system.
Troubleshooting Steps:
1. Try a spare key: If you have a spare key, try it. If this solves the problem, it points strongly to a key or Passlock issue.
2. Check the Battery: Though less likely to be the primary problem, make sure the battery terminals are clean and tight. A voltage test will tell you the battery's condition.
3. Jump Start (with caution): Attempt a jump start. If it starts and runs briefly before dying again, this could confirm a weak battery exacerbating a deeper problem. However, don't continue to jump start it repeatedly if it keeps dying—this could cause further damage.
4. Professional Diagnosis: This is the most crucial step. A mechanic with access to diagnostic tools can check the Passlock system, the BCM, the security system module, and the wiring to pinpoint the source of the problem accurately and safely.
In short, the red security light is a strong indicator that the anti-theft system is at the heart of this issue. Don't attempt extensive DIY repairs without understanding the electrical systems; incorrect actions could cause further damage or even injury. A professional diagnosis is the recommended path.
