Symptom · Any modern vehicle with a CAN network
Two different faults look identical from the driver's seat: a battery that no longer holds charge, and a circuit that stays awake and drains a good one. Test them in that order, because replacing a healthy battery is the classic apprentice invoice.
Battery at end of life
It cranks a warm engine and still fails a load test, so it looks fine until it sits.
A module refusing to sleep
Infotainment, gateway or a body module stays awake and pulls hundreds of milliamps all night.
Aftermarket accessory wired to permanent live
A dashcam, tracker or amplifier fitted to a constant feed never switches off.
Boot, bonnet or door switch stuck
The car believes a door is open, keeps lights and modules awake, and never enters sleep.
Failing alternator or charge control
It never fully replaces what starting takes, so the battery slides down over days.
Chafed wiring shorting intermittently
A rubbed loom against a bracket drains through the bodywork rather than through a load.
Nothing on that list is the answer yet. A measurement decides which ones survive.
Test 1
Measure resting voltage after the car has stood for at least four hours untouched, then load test the battery.
A battery that fails the load test is your fault, full stop. One that passes means you are hunting a drain.
Test 2
Check charging voltage at idle with lights and blower on.
Below about 13.5 V, the car is not replacing what it uses and no drain hunt will fix that.
Test 3
Clamp an ammeter around the negative battery cable, close everything up, and wait for the car to sleep. Do not open a door.
A current that never drops is a module staying awake. A current that drops and then wakes on its own points at a switch or a network request.
Test 4
With the drain present, pull fuses one at a time and watch the clamp reading after each.
That fuse names the circuit. Nothing found in the interior box means you have not waited long enough for sleep.
Test 5
Check every door, boot and bonnet switch state on the scan tool, then close each one and watch the state change.
A switch that reads open with the panel shut explains a car that never sleeps and needs no fuse pulling at all.
Load test first, charging second, and only then the clamp. Skipping the first two is how a good battery gets replaced twice.
Open a Case with the vehicle and the symptom in front of you. It asks the two or three questions that narrow yours, then names one test with the value to expect, and re-ranks the list on what you measure.
Open your first CaseFree and complete. No account, no card. A Bay Pass after that is $12.00 for 90 days.