Most vehicles have a 12-volt battery that provides power to start the engine. The battery is recharged by an alternator and requires a constant 14-volt input while the engine is running. If the engine does not start after remaining parked overnight, you could have a problem with the alternator, the regulator or the battery. Test the voltage regulator to determine if the battery is receiving the exact voltage required to properly recharge while the engine is running. If it isn't, you need to adjust the regulator.
Raise the hood of your vehicle and prop it open. Set the parking brake and put the shifter in Park (automatic transmission) or neutral (manual transmission).
Disconnect the negative (black) cable from the battery, followed by the positive (red) cable. Attach the black multimeter probe to the negative terminal on the battery. Attach the red multimeter probe to the positive terminal on the battery.
Start the engine. Turn on the multimeter and move the mode selector to the "VDC" (Volts Direct Current) position at the 20-volt range.
Read the multimeter. If the voltage reading is lower than 14 volts, you may need to replace the alternator/regulator and possibly the battery. If the display reads higher than 14 volts, the voltage regular needs to be adjusted or replaced.
Remove the cover of the voltage regulator, which is on the alternator. Locate the regulator adjustment bolt on the alternator. Use a Phillips screwdriver to move the adjustment bolt until the multimeter reads 14 volts exactly, then reattach the cover.
If there is no adjustment bolt on the alternator for the regulator, you need to replace the alternator/regulator.