Battery drains overnight (parasitic draw)
Car won't start in the morning — battery is flat. How to find the parasitic drain using OBD-II and a multimeter.
Parasitic drain is electrical draw from the battery with the car off. Normal is under 50 mA. Over 80-100 mA and a battery goes flat in 2-3 days.
Common culprits: (1) aftermarket alarm or immobilizer module stuck awake; (2) BCM hanging in a wakeup state; (3) USB chargers or dashcams left powered; (4) faulty fan relay or A/C compressor relay staying energised.
OBD-II is limited with a dead battery — but after charging, read B and U codes (BCM/network). This often points to the module that won't sleep.
First diagnostic steps
- Charge battery, then check resting voltage (should be 12.5-12.7V)
- Connect a multimeter in ammeter mode between the negative terminal and ground cable
- Wait 10-15 min for all modules to sleep (normal < 50 mA)
- Pull fuses one by one — when current drops you've found the circuit
- Read B/U codes via OBD-II for a hint on the sleeping module
Likely fault codes
Click a code for detailed description, causes, and diagnostic plan.