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The car: 2006 Subaru Legacy Touring Wagon 2.0 GT (Japanese model) - recently purchased; warranty does not cover this

The situation: Something happens with the 12v supply and once the car is stopped (switched off) dash lights won't come on, central locking won't work, car won't even try to start. No door chimes when key in/door open, etc. Battery OK. Seems to come back after some time. See this video of nothing happening when I turn the key.

Detailed: On Friday I was driving to work and my dashcam shut off, so I pulled over to check it. After checking it was all connected OK (via USB adapter to the 12v socket), I tried to start the engine but nothing happened. Recovery came out and got me started with a jump start pack (although the battery was showing 12v before). On the recovery guy's advice I replaced the battery.

This morning a similar thing happened (dashcam and FM transmitter cycled on and off a couple of times shortly after I started driving), except after ~25-30 minutes of the key in the ignition in the off position, I opened the door and the door chime came on and the dash was once again alive. I turned the key and the engine started immediately. I shut it off, turned it back on, and again it started immediately.

When this happens I can still move my seat (motorised) without issues.

Probably unrelated but: I've currently got a P0410 engine code (secondary air system) showing, which is booked into Subaru for repairs later this month. Hopefully this isn't a red herring for some other electrical system problem.

Also probably unrelated: but I very very regularly get static shocks off the car when leaving it, and when this happens I get a static shock even if I just sat in the car for a minute or two. This could always be the case, since I usually just get in and drive and then experience it upon getting out, but I'll add it here just in case.

The question: What steps can I take next to start diagnosing this? I'm not extremely experienced fixing cars, but I'd like to get a bit more hands on and I do have an electrically knowledgeable friend to help.

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    Are you saying that when everything else is dead, you can still move the electric seat?
    – HandyHowie
    Nov 17, 2019 at 11:29
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    @HandyHowie Yes, that's correct.
    – ProgrammingLlama
    Nov 17, 2019 at 12:51
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    I am wondering if perhaps the battery in the key needs replacing - maybe someone else could say if it's worth trying.
    – Andrew Morton
    Nov 17, 2019 at 17:43
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    2006 is awfully old for a factory warranty. Is this a financed deal? If so get out of the note ASAP. Don't owe money on a car that's out of warranty!
    – Harper - Reinstate Monica
    Nov 17, 2019 at 19:56
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    I had a puzzling similar issue, except where my engine would completely drop out while driving, and no one could figure it out (dealer said ignition switch, changed it, car died on the way home!). Turned out it was a failing Main Relay, which I replaced myself, completely solving the issue. Might be a possibility, and fairly cheap and easy to dyi. Google some YouTube videos to decide if it's worth trying.
    – Jeff Y
    Nov 18, 2019 at 18:46

6 Answers 6

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You may have a loose or corroded engine ground connection. The next time you have the electrical failure, leave the ignition switch ON and, using a voltmeter, measure voltage between the negative post on the battery and any shiny clean metal part that is bolted to the engine block. This should always read zero volts, or nearly zero. If it reads something like 12 volts, you have a bad engine ground which must be repaired.

EDIT: To diagnose a bad engine ground without a voltmeter, you can use a jumper cable. The next time you have an electrical failure, connect a single jumper cable to the negative battery terminal (first) and a shiny clean metal part that is bolted to the engine block (second). Do not connect the second jumper cable of the pair to anything. If the car starts and everything works, this confirms the bad engine ground. For safety, keep the cable clear of moving parts such as belts, pulleys and fan blades, and do not drive with a jumper cable attached.

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    Yes i've seen this, it can cause very weird behaviour! Also, basically ALL the diagnostics in your car just assume the ground is good.
    – Level River St
    Nov 17, 2019 at 22:13
  • Would a high frequency of static shocks from the car also point to a bad ground?
    – ProgrammingLlama
    Nov 18, 2019 at 0:55
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    The car's carpet or upholstery is much more likely the source of the static shocks. Static shocks are low current but high voltage, usually hundreds or thousands of volts. Car electrical systems are too low a voltage to contribute much to such a shock.
    – barbecue
    Nov 18, 2019 at 5:05
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    +1 This is what happened to our 2005 Odyssey. Exhibited trouble starting. Then it would start just fine. Then it would trouble again, even though the battery was fine. It was a corroded engine ground cable.
    – G Trawo
    Nov 18, 2019 at 15:20
  • I expect this is the most likely next step for diagnosing the issue, which is ultimately what my question was asking for. Thank you.
    – ProgrammingLlama
    Nov 19, 2019 at 2:34
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Since you lost power to your 12v accessory socket at the same time as the other systems, I can’t see this as being an computer malfunction. This sounds to me like you have a fault in your main fuse box which is taking out a number of circuits.

There may be a bad connection of one of the main fusible links. Unbolting them and cleaning may fix it. You may need a circuit diagram to identify the exact fault, or you could just clean them all. Make sure you disconnect the battery before doing this.

Alternatively, are there a number of large wires that go to the battery 12V post? If so, it may be one of them that is bad.

If you add a photo of the battery connections and one of the main fuse box it may help.

Another possibility is the ignition switch could be faulty. Quite often power to the seats is delivered as soon as the doors open and do not need the ignition switch to be on.

Can you identify all if the circuits that work while the fault occurs, for example try all of the different lights?

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    Agreed. Check all the grounds as well. Especially any you might have messed with.
    – mreff555
    Nov 17, 2019 at 16:52
  • @Handy I'll have to take a look at the fuse box. I wish it could be the ignition switch, but if that were the case things like central locking, interior lights, would still function. Thank you.
    – ProgrammingLlama
    Nov 18, 2019 at 0:48
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Welcome to the site, and thanks for putting in a good amount of details. The fact that you have replaced the battery and your seat motors still work when you have the issue eliminates a bad battery or main battery connection. The symptoms sound a lot like a computer issue to me because the computers control the dash lights, ignition, either the computer isn't getting power or it's malfunctioning. The fact it comes back after a time means it could be heat related.

If it's a bad computer that's not really something I'd advise an inexperienced amateur to fix, but you could quickly and cheaply replace the main relay, which could very well be your problem. The position should be somewhere in the passenger side under the dash, but this varies from model to model. Note, that's not the main fan relay, but the main relay. Also check the fuse that supplies the computer to make sure it is seated properly and has a good connection. If that doesn't work you and your friend could use a multimeter to test the connectivity to the ECU (Engine Control Unit).

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  • This does not eliminate the battery connection completely. It eliminates a loose connection, but it does not eliminate an intermittent cable. I saw a very similar problem that did turn out to be an intermittent primary cable.
    – John R. Strohm
    Nov 18, 2019 at 19:56
  • So it happened again today. Hazard/warning lights work, parking lights work, horn works. Hazard light arrows even flash on the instrument panel. Voltage between battery negative and the body, engine, etc. is 0. Jump starting from another vehicle doesn't work, with the negative connected to the battery or the body. Car was working fine this morning, and even the ABS warning light that came on yesterday was gone. Do you reckon this points to the main relay being the fault?
    – ProgrammingLlama
    Nov 19, 2019 at 10:06
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I have no idea of where in the car to look for it but this sort of behavior sounds thermal to me. Something delivers power when it's cool but goes open circuit when it's hot.

A couple of thoughts on this, I don't know how practical given the situation:

1) When it's failing look over the circuit with a multimeter. I would connect the leads together through say a 200 ohm, 1 watt resistor--enough to ensure there's more than a trace of power reaching the point. (And this would be even more informative if you see 12v without the resistor but it drops with the resistor.)

2) Disassemble/clean/reassemble junctions.

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  • I have been wondering about that. When it was cold this morning it wouldn't start at all, but at lunchtime I went home to it and the car did start. The previous times were also when both the outside temperature and engine are cold. Though after a short drive, it did the 12v thing again and started showing the ABS warning light, which is new. I'm beginning to think it might be best for me to just take it to a mechanic who specialises in electrical issues.
    – ProgrammingLlama
    Nov 18, 2019 at 5:20
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I had a similar problem with my car late last year. The car will go totally off and few minutes later turn-on the dashboard again. After few days of struggle and investigation asking battery guys and electricians for help, I discovered that the cables to the battery terminal were loose. The problem totally went away after tightening them.

Hope this helps...

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I had a similar problem with a '77 Chevy Impala station wagon. It drove me crazy. I finally managed to isolate it to the primary cable from the battery to the starter solenoid.

After replacing the primary cable, I removed the battery post clamp from the old cable, and found myself staring at a handful of wire fragments. The outer layer of cable wires had all broken, causing the primary cable to become intermittent.

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