U3014 indicates an open circuit in the 'D' input power feed to a control module, meaning the module is not receiving the battery voltage or ignition power it expects on that specific circuit. This is a power-supply fault rather than a data-network fault, and the module that logged it may be the ABS module, airbag module, body control module, or another module depending on the vehicle architecture.
The power feed for this circuit normally originates from a fuse or fusible link. A blown, loose, or corroded fuse is the single most common reason an entire module power circuit goes open.
Chafing, pin-pull at a connector, rodent damage, or previous repair work can create a break in the B+ or ignition-power wire that feeds the module on circuit D.
Terminal fretting, water intrusion, or spread connector pins can raise resistance enough that the module sees the circuit as effectively open, even if some voltage is present.
If circuit D is switched through a relay or the ignition switch, worn relay contacts or a failing switch output can interrupt power and set this code.
This is uncommon. The module itself can set the code if its internal power-monitoring circuit detects an open, but the external wiring and fuse should be ruled out first.
It depends on which module lost power. If it is a non-critical module such as a body control or convenience module, the vehicle may still drive safely. If it is the ABS, airbag, or engine-control module, the affected safety or drivability system may be disabled, so it should be diagnosed promptly.
Usually not. U3014 describes an open in a power-supply circuit, so the fault is most often a blown fuse, wiring break, or connector problem. The control module itself is rarely the cause and should not be replaced until the external power circuit has been proven good.
When a module loses its power feed, it drops off the network. Other modules then log communication-fault codes because they can no longer talk to it. Fixing the power circuit typically clears those related codes as well.
Manufacturers label separate battery and ignition power feeds to a module as Input Power A, B, C, D, and so on. 'D' is simply one specific power feed pin; the exact function is defined in that vehicle's service information.