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Can't connect to ECU anymore


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So basically I had my 2004 SG XT Forester sitting and my father tried to move it but ended up swapping the active and negative on the battery. The 100A main fuse blew and the car wouldn't do anything so I replaced that with a new one. the car then would now turn on, however the fuel pump wont stop priming, when cranking the car struggles to start and then when started the car runs horrendously. all the while I'm unable to connect to the ECU which is a G4x WRX104X Plugin. I have looked at all other trouble shooting, So to answer the questions you have. Yes I used to be able to connect to the ecu, the car was driving prior. there is a bright blue light on my ecu when ignition is switched to the on position (it disappears when cranking), yes I've changed the comport around, Yes it does still appear in my laptops connected devices. and yes all software has been uninstalled (drivers included) and reinstalled onto my laptop.

is my ecu fried? There was no visual signs of burn or scorching, the only thing that has me worried is a funky smell when opened up from the case.

If anyone I could get some help with what I should do next or any other solutions that would be amazing.

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Your symptoms sound very much like the ecu is not powered up.  When you have an aux device that is powered up, but the ecu is not (fuel pump relay for example), then current will flow through the relay coil, into the aux, through the flyback diode and onto the 14V bus inside the ecu.  So this would make the LED light up and the fuel pump relay engage.

The reason for not powering up however will need a bit of investigation - it could be something as simple as another blown fuse or a damaged wire or damaged ecu.  

If you have a multimeter I will suggest measuring voltages in a couple of places.  The positions that we need to test will depend on what the 2004/2006 jumpers are set to - so I hope I can explain clear enough. 

We want to do these measurements with the ecu unplugged and we will just be measuring at the terminals in the main loom plug:

  1. First we want to check there is an ign switch signal.  This pin should measure ~+12V with the ign switch on.  If your jumpers are in the 2004 position then the ign switch pin is D15, if the jumpers are set to 2005/06 then test D14.
  2. Assuming that looks good we next want to check the main efi relay coil is powered, This is pin D17 for 2004 jumpers or D16 for 05/06 jumpers.  This pin should have ~12V at all times, ign switch on or off.  
  3. Provided test #2 looks good, now we want to ground that same main relay coil pin that we just tested (D16 or D17), short it to ground with a paper clip or small piece of wire etc, if you hear the main efi relay click when you short this pin then that would suggest that is ok.
  4. With D16 or 17 still shorted to ground we now want to test the main ecu power supply from the main relay contacts.  You should have ~12V on pin B5 & B6 if this is ok.  

Report back what you find with those tests and I will try to interpret what I think is the problem.


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Thanks for the quick reply Adam.

I just went and looked, my jumpers are set in the 2004 position so I went and tested D15, with no luck wasn't reading a Voltage at all. I moved on to testing D17 just to make sure I wasn't doing something wrong and did read -11v from the positive terminal of my battery to that pin. couldn't get the coil to click or read any voltage on b5 or b6.

any ideas?

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If you aren't getting voltage to your D15 pin you need to go further up stream in the power supply to the ecu from the ignition switch and find where the voltage stops.  Pre-emptively I would go to every single fuse box on the car and pull and verify each fuse is intact and not blown.

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8 hours ago, Izzy said:

and did read -11v from the positive terminal of my battery to that pin.

That sounds possibly like you are measuring wrong.  One probe of your multimeter should be on ground or bat negative and the other measuring the specified pins.  

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So I've finally had a moment to double check the pins readings, I was wrong it's in the 05-06 layout, so redoing those test I was getting the results you would expect to get. 

D14 I was getting +12v with ignition on
D16 had +12v all times
Grounding out d16 the relay clicked
and while D16 was grounded out B5 and B6 both had 12V


I also went ahead and looked at every fuse in the car, the only other fuse that was blown was a 15a fuse in the back of the headunit which I replaced. other than that everything else was the same.

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