2007 BMW x3. No crank, no start. Battery voltage currently only 11.5. P0012 code. Nothing else checked. Have multimeter

hello

If the car's not cranking then first check it can turned over by hand by using a socket on the crank pulley nut, so double checking for any mechanical issues

If OK then check that the main battery cables are on tight and the terminals are clean and bright. Do the same for the engine / gearbox earth strap.

Then follow the main positive lead back to the engine and down to the starter motor, and check the connection at this end as well.

There should also be a small wire to the motor which should have 12V when you're turning the ignition switch / pushing the start button to crank. If the 12V isn't there, check back along this wire for damage to the insulation or corroded connections

This 12V feed comes directly from the ignition switch / starter button so this may be faulty and need replacing

Check the resistance of the main positive lead that runs down to the motor as anything more than a few ohms and it should be replaced

Don't forget to try a jump start from another car / battery as its possible the battery is poor, if you have a voltmeter a healthy battery should read 12.7+V

also try substituting the battery for another as I've known a very flat battery to drag down the system voltage too far even with jump cables

one other thing to consider is that the immobiliser is faulty and preventing the starter from being enabled - however if the dash / ignition lights are on then this usually means that its off

If this is all OK then its possible the car needs a new starter motor

I would check the ground cable from the engine to the body it is a common failure point because it is just exposed bare copper wire. it is under the car on the passenger side of the engine near the back you can see it from under the vehicle.