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Why does my 2006 Subaru Outback go dead after being undriven for a week or more? Mechanics stumped

My 2006 Outback starts and drives great until it’s been parked for a week or more. Then I can’t even open the doors with the remote much less start it. It is totally dead! Mechanics are stumped. They have checked the circuits. Put a new battery in about 4 months ago. finally took it to a Dealership. Condescending guy assumed I had left an inside light on But that’s not the case. Now its sitting in his lot for a week so he can see for himself. This has happened SIX times in the last year and it’s driving me nuts. Help!

More than likely there’s a parasitic current draw on the battery as the vehicle sits. This can be caused by an electronic module that fails to shut down or go to sleep.

In the past one would disconnect the negative battery cable and then connect a test light/meter between the cable and the battery to check for a current draw. But if there’s a module that is failing to go to sleep that’s drawing the current and you disconnect the battery, this forces the offending module to go to sleep, so when you go to check for the current draw it’s not there.

There are a few ways to test for a current draw without disconnecting the battery from the vehicle. The easiest way is to install a battery disconnect switch such as this

Then you connect a meter to each side of the switch, open the switch and the battery is still connected to the vehicle thru the meter. Now you can start pulling fuses/relays to determine what circuit is drawing the current.

Tester

You need a new mechanic! There is a relatively easy way to check this. You measure the ‘off’ current draw from the battery. If it is above the maximum allowed, then you start pulling fuses until you see it drop. This isolates the problem to a single circuit. Then you troubleshoot the things on that circuit.

I did have a draw on my battery that was hard to find. Turns out the battery was leaking a little, and the fluid was making a poor connection to ground. Replaced the battery under the battery warranty.

The Subaru spec for the residual current when everything is turned off, all doors properly closed, key out etc, is 150 mA (0.15 A) maximum. Typically, the measured residual current is far lower.

Subaru refers to the residual current as “dark current”. Dealer should be testing the car according to the method outlined in TSB 07-62-07.

The battery was replaced, presumably with a new one, and it’s less likely that the new one would have the same fault, such as internal leakage, as the previous one (although it’s not impossible!).

At the maximum 150 mA, it is possible for a battery to be run down while parked. A typical fully charged battery has a capacity of about 50 Ah, and could be discharged in 13 - 14 days. A battery that isn’t fully charged when the car is parked will become discharged even earlier. At the normally lower dark current rate, the battery should last a lot longer, but a poorly charged battery could still be depleted in a week’s time.

The problem was apparently experienced a number of times, but does it always go dead after a week or so, or does it happen only some of the times the car is parked for an extended period? If there are instances where it did not go dead, then it’s intermittent. That would be more difficult to find, and a real challenge for any mechanic. Hopefully it will happen with the car at the dealer and they will be able to take it from there.

Do you have a remote car starter? These are infamous for increasing parasitic draw as they need to remain “alive” to pick up your remote start signal. People who drive cars infrequently and have remote car starters are more likely to experience problems like the one you are describing. I’m just throwing this one out there, even if it is not the case for you.

Thanks everyone for your comments. To the person who asked about always/intermittent – it always happens when the car is parked for a week or more. I go out of town a lot, and my remote key is usually with me, so not sure about the draw there, but all the comments were helpful. Happy driving!