Try booting up your JTAG without the HDD. If it then works just blow away the dust in the plug, plug it in then smack it(Not too hard)and it should fix. If not, then try taking out EVERY thing plugged into your JTAG leave for an hour, then it should work.

1 red ring on the right stands for harddrive failure

dreadsxsv wrote wow, you all FAIL! e74 is fixed by replacing the heatsink compound between the heatsinks and cpu/gpu, do them both.

But it can also be other problems.

Like others have said, try it without the HDD, unplug all cables and leave it for a while and try it. If that doesn't work;
1. Make sure none of the Jtag wires are touching either each other, or something else metal (DVD Drive) etc., as that will cause the console to think it has an E74 error.
2. Make sure you haven't got a loose or cold solder point on where your Jtag wires are, as that again, will cause an E74 problem. If you do, resolder properly and you should be good to go.

If you've tried all of those, you will probably need a proper reflow or reball. So send it off to somewhere/someone that knows what they're doing and has the proper equipment to do so. I don't know what prices are like for it in the US, but in the U.K. you're looking at about 40-50GBP.

Hope this helps.