Your problem may be elsewhere... Because the way most PSUs are designed (to save cost), they are not capable of outputting their full rating on the 12V rail unless the 5V rail is also adequately loaded. This has been causing some problems in standard system with high-end graphics cards too, where the 12V load far outstrips other loads.
Still, I find it odd that it stays on and you're reading 12V. The problem may be with your m1atx. Does it still work off your car batt?