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Intermittent ECU Problem - Tough to Fix

Hello, everyone. I’ve had this weird problem with my 2006 Forester for years, and I’ve finally figured out that it’s caused by the ECU. In a nutshell, my car won’t start the day after I drive it in rain (good thing I live in Arizona, right?).

Once it dries out, it starts again. Until then, it acts like I’m trying to start it without putting the clutch in. As you can imagine, this is really hard to replicate for a mechanic. And when it happens, I’m at home. I’d really like to solve this problem because I’m planning to sell this car in the next few months. Any suggestions?

Water leak over the ECU? Water leak on clutch switch? Evidence of water leak anywhere? Why fix it now? Just leave a note for the new owner.

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It’s tough to tell. My windshield has a crack, but I had this problem before the crack. I opened the ECU for a look, and it didn’t exactly look like the engine room of the Titanic, post iceberg. But I don’t know exactly how much moisture it takes to make it malfunction. Could just be traces.

That’s an interesting idea about the clutch switch, though. Thanks for that thought!

If I sell it, I really wouldn’t feel right unless I disclosed the problem. If I’m the buyer, that’s a major red flag. I’d rather take care of it beforehand if it’s feasible. The alternative is just to trade it in and let the dealer worry about it! :smiley:

So this is a no-crank problem right, you don’t hear that rrr rrr rrr souind when you turn the key to “start”. And I presume you don’t hear any click sound either. tjhat would be what happens if you forget to push the clutch in, no rrr, and no click.

hmmm … is there a reason you think this is the ECU that’s the problem? B/c I wouldn’t guess that, not as my first guess anyway. What you need to do – or have done – is trace out the start signals that get sent when you turn the key to start. the signal usually goes from the battery to the ignition switch, to the clutch safety switch, often to a small under-dash relay, to the ECU, then to the starter solenoid (the small wire attached to the starter motor). You’ll need a wiring schematic diagram, then have somebody knowledgably trace it out point by point until you discover the first place where the signal isn’t getting to.

You indicate above when you looked at the ECU it was a little damp. Where is the ECU located in your vehicle, inside the engine compartment? If dampness is the cause of this problem most likely it’s just a connector that connects to the ECU that’s a little corroded. I can’t imagine a design where the ECU circuit board could get damp just b/c of rain, unless there was some kind of crack or fault in its covering, etc.

PS, check any fuses that are involved with that circuit, especially if any have that same dampness problem.

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From your description of the problem it seems that the starter motor doesn’t work when the problem happens. If so, the ECU most likely isn’t the cause of the trouble and is most likely being caused by the clutch safety switch as @Bing pointed out. You should verify that power is getting past that switch while the trouble is occurring. There could also be a start relay and security relay involved with this issue. Another possible problem is the starter solenoid contacts, which wear out over time. If you hear a fairly loud click when you have the trouble then suspect that is the cause. The car is at the correct age for that kind of problem to happen.

Since the ECU is pretty well sealed for water to get into the unit it is hard for me to imagine that water has really gotten inside it but if you saw signs of water damage inside it then that is not good at all. You can get rebuilt ECU’s if you really need one but I don’t think that is going to fix this problem you are having. Looking at some data there may be some communication link between the starting system control, the dash cluster, and the ECU, so the ECU does seem to have control on the starter motor action but again, I doubt it is the real cause of the trouble.

The clutch switch and starter solenoid contacts are my best suspects for this trouble.

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I actually didn’t see any signs of water damage. My comment on this was a bit unclear, sorry! I just had a few different Subaru enthusiasts making the ECU their first guess.

It’s odd because I had the clutch switched replaced shortly after this problem showed up. It still is really weird that it only happens after getting wet, isn’t it? Thanks for the advice!