A parasitic drain is when something keeps drawing power after the car is off and locked. Healthy modern cars settle to under 50 mA after about 30 minutes; anything over 80 mA will flatten a battery in a few days. This is the same test every shop uses, and you can do it in your driveway with a $30 multimeter.
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A switch that does not detect the lid closing leaves a bulb on after you walk away. Often the easiest catch - listen for the relay click or watch for current to drop when you push the offending switch by hand. Cost: $0 - $50. DIY: Easy. Severity: Low.
Get a Free AI Diagnosis →Dashcams, amps, remote starters, and trailer brake controllers wired to the always-hot side of the battery instead of switched power. Pull each one out of the circuit to confirm. Cost: $15 - $200. DIY: Medium. Severity: Medium.
Get a Free AI Diagnosis →Body control module, radio, infotainment head unit, or telematics module that stays partially awake. Often shows up as a high steady draw that does not drop after 30 minutes. Diagnosis usually requires a scan tool. Cost: $150 - $400. DIY: Hard. Severity: Medium.
Get a Free AI Diagnosis →A failed diode in the alternator lets current flow backwards into the alternator with the engine off. Easy test - disconnect the alternator output lead and recheck the draw. Cost: $350 - $800. DIY: Hard. Severity: Medium.
Get a Free AI Diagnosis →A welded or stuck relay keeps a circuit live (fuel pump, cooling fan, headlight relay are common). Pull relays one by one with the meter inline; the bad one drops the draw. Cost: $10 - $60. DIY: Easy. Severity: Low.
Get a Free AI Diagnosis →If your scanner shows one of these codes along with the symptom, run a free AI diagnosis to confirm the root cause.
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Under 50 mA (0.05 amps) once all the modules have gone to sleep, usually 20-30 minutes after locking the car. Some luxury cars run higher (up to 80 mA) due to always-on telematics.
Set the meter to 10A DC. With the car off and locked, disconnect the negative battery cable and connect the meter inline between the cable and the post. Wait 30 minutes for modules to sleep, then read.
Modules stay awake briefly after locking the car (security check, radio memory, etc). They time-out and go to sleep over 20-30 minutes. Testing before that gives a false high reading.
With the meter still reading high draw, pull fuses one at a time. When the draw drops to normal, that is the circuit. Then check what is on that circuit and chase the part.
Yes, and it is easier - no need to disconnect anything. A DC clamp meter that reads down to 10 mA (most do) gives the same answer faster.
Typical diagnostic time is 1-2 hours at $120-$180/hr, so $120-$360. Plus the parts to fix whatever is found.