I would consider it more likely that the lower voltage produced by a cold battery, added to the higher current draw required to spin an engine with cold oil, means that one of the critical relays may not be fully energized, so either the ignition (Relay XVI), the injection (Relay XXV) or the fuel pump (Relay XX) is not being energized.
My first suggestion is to fully charge the battery, and try to crank the engine while the battery is warm from charging. If the engine runs, you can try again after the battery has chilled overnight.
If the engine still won't run, use a mechanic's stethoscope to listen for the injectors clicking while someone cranks the engine; and use an inductive-pickup timing light to check for ignition on each of the plug wires.
If you have working injectors, a hot spark, and fuel pressure, the engine should run.
Listen under the rear bumper while someone hits the starter. There should be a two-second buzz from the fuel pump. If there is not, Relay XX, the fuel pump relay, may be bad.
If the fuel pump buzzes for two seconds, get a four-foot piece of rubber or plastic tubing. Stick one end in your best ear. Put the other end on an injector while some one cranks the engine. There should be a click-click-click as the engine turns over. If there is no clicking, Relay XXV, the injection relay, may be bad.
If the fuel pump buzzes, and the injectors click, then try hard to borrow
an inductive-pickup timing light from someone who used to work on cars.
Hook the power leads to ground and to the jump start terminal under the hood. Hook the pickup around the coil wire. Pull the trigger and check for a flashing light while someone cranks the engine. If there is no light, the timing light may be bad - check it on another car. If the timing light works, but you get no flashing on your 928, Relay XVI, the ignition relay, may be bad.
The problem may not be a relay - but relays are the most common cause for a 928 not running. You can substitute the horn relay, or the back-up lamp relay for any of the critical three, even though one of them may have the fuse tester built in and will look different.
If you don't have the relay chart that should be in the clear pocket on the
central power panel door, download and print the one from our web site at
www.928gt.com . Spray it with some type of durable clear coating, since
ink-jet ink is not waterproof.
The relays ARE hard to remove. Porsche makes a special tool to pull them, but I'm not impressed. It is just a special pair of pliers that will cull the cover off of the relay.
One possible way of getting a relay out is to go to an auto parts/tool store and buy a spark plug boot remover. This is a black metal strip with a finger hook on one end and a right-angle on the other end. Grind part of the right angle end off. Disconnect the battery ground strap. Stick the shortened right-angle under the base of the offending relay and pry and pull.