4Runner Undervoltage Issues

The symptoms you describe strongly suggest a charging system malfunction. Verify the battery's state of charge and conduct a load test. Scrutinize the alternator's output voltage under varying electrical loads. Examine the battery terminals for corrosion and ensure a secure connection. A parasitic draw test might also reveal an unexpected current drain when the vehicle is at rest.

Thanks for the quick reply! When you mention checking the alternator's output voltage, what's the best way to do that? And what voltage range should I be looking for to know if it's working correctly?

The alternator's output should be assessed with a digital multimeter while the engine is running. A healthy charging system will typically exhibit a voltage between 13.5 and 14.5 volts. Deviation from this range indicates a potential alternator issue or a faulty voltage regulator. Given the complexity of modern vehicle electrical systems, I recommend consulting a qualified technician to accurately diagnose the root cause. Ignoring the issue may result in further damages.

Thanks for the advice! I took it to a local workshop, and they confirmed it was indeed a defective battery. Replacing the battery solved all the issues, and it cost me 290€. Appreciate the help!