2003 C240 (W203) battery drains in 2 to 3 days. There is a constant current drain of 0.4A on 12V battery. The car is USA model. I have new alternator, new battery, new alarm siren (as it contains its own battery, which if dead can cause drain on main battery). Over a week, I have tried to locate the problem by removing and re-inserting fuses one-by-one in all the 3 fuse boxes (under the hood engine area, driver side dashboard side, and in the trunk) but none of the it cause the current to drop below 0.4A. Below is what i have done so far.
- Before checking fuses, I made sure to close latches on all doors and pushed the door close push pins on the body with clamps. I also, closed the latches for the hood cover and truck.
- Battery is fully charged before checking for the cause of the drain current. Voltage is about 13.1V.
- My clamp meter is on battery negative cable connected to the body. I reset the reading to 0 before measuring the current, which fluctuate from 0.3A to 0.6A, but stabilizes around 0.4A.
- Car alarm is active except for when I remove the fuse for it, in which case I had to reactivate it after inserting the fuse back.
- I have removed the passenger and driver seat power cables, one at a time as well as for both the seats, but this also did not drop the current.
- I have repeated above process 3 times over a week just to make sure I did not miss any of the above described steps or fuse.

What else can I check? The new battery has discharged twice due to this issue and I had to use jump starter to start the car. The charged battery drains in 2 to 3 days and will not even charge with a 3A 12V trickle charger unless I first use jump start and bring the battery voltage to health level and then only trickle charger starts charging the car battery. Now I am taking no more chance and keeping car hooked to trickle charger when parked at home. Any suggestion is appreciated.

 

Any aftermarket electrics fitted?
does interior bus go to sleep?

 

This got resolved. The clamp meter was not accurate as it was showing 0.42A current even on the disconnected negative terminal of the battery. As per the advice of slammer111, I used a digital multimeter with a series connection to measure the current and found that the front passenger seat was draining over 900mA as the meter was showing 980mA with the power connector inserted below the seat and 77mA when this power connector in pull out. This suggests the passenger seat controller is bad which I hear costs about $1K to replace. Since my car is 19 years old, right now, I just pull out the passenger seat power connector. . I plan to cut the battery power green-red wire and connect it to the engine-ON power pink-black wire using a blue tap connector. These wires are on the passenger seat power connector below the seat.