Here are some possible causes:
* Alternator Failure: This is the most common culprit. The alternator might be failing to generate sufficient voltage, or it could be completely dead. A faulty alternator could have problems with its diodes, stator, or rotor.
* Alternator Belt Issues: A slipping, loose, or broken serpentine belt (the belt that drives the alternator) prevents the alternator from spinning properly and generating power.
* Battery Issues (despite reading 11.9V): While the reading suggests a charging problem, a weak or failing battery could still be a contributing factor. A battery with internal problems may not be holding a charge well, even if the alternator is working to some degree.
* Wiring Problems: A loose or corroded connection in the charging system wiring (between the battery, alternator, and voltage regulator) can prevent proper voltage flow.
* Voltage Regulator Failure: The voltage regulator controls the output of the alternator. A faulty regulator can lead to undercharging. This is usually part of the alternator assembly on newer vehicles, so often requires replacing the alternator.
* Parasitic Drain (Less Likely): While a parasitic drain will discharge the battery when the car is off, it's less likely to be the sole reason for a constant 11.9V reading *while the engine is running*. However, a very large drain *could* overwhelm the alternator's ability to charge.
What to do:
1. Check the battery terminals: Look for corrosion. Clean them thoroughly.
2. Inspect the serpentine belt: Check for cracks, fraying, or slippage.
3. Have the alternator and battery tested: A professional mechanic can perform a load test on the battery and a test on the alternator to determine if either is faulty. This is the most reliable way to diagnose the problem.
Don't continue driving the car with a consistently low voltage reading. Driving with a constantly undercharged battery can damage the electrical system and leave you stranded.
