P0318: Cadillac no-crank faults that point toward ECM power and communication

P0318 should not automatically lead to rough-road sensor replacement when a covered Cadillac will not crank or will not release the ignition key. GM PIC5460B describes those symptoms with several ECM faults, including P0318, and possible communication codes in other Class 2 modules. Its diagnostic focus is the ECM’s ignition feed and Class 2 wiring.

Start with the shared electrical condition

The bulletin distinguishes the ECM’s reported ignition-relay feedback from voltage actually available at the ECM fuse. If the feedback is below battery voltage, the workshop checks supply at the underhood fuse and whether the relay is being commanded correctly. Battery voltage at the fuse does not prove that the same feed reaches the ECM; the circuit between them still needs evaluation.

A second branch follows if the ignition feed is sound: investigate a possible open Class 2 connection to the ECM. Power delivery and network continuity are therefore separate checks. This sequence is more specific than assuming that the many stored codes represent many unrelated component failures.

Why intermittent history matters

GM notes that the relay or the listed circuits can have intermittent connection problems. Only after wiring and connections check correctly does the bulletin suggest a relay or ECM concern. It does not authorize immediate ECM replacement from P0318 alone.

Record whether the key is trapped, whether the engine cranks, and which modules remain accessible during the event. Preserve the complete fault set before clearing it. The cited case is limited to the stated CTS and SRX years with the LY7 engine; other vehicles or a P0318 without this symptom pattern require their own diagnostic information.

Source and applicability

GM PIC5460B, March 2016: 2004–2007 Cadillac CTS and 2004–2009 Cadillac SRX with the 3.6L LY7 engine.

PIC5460B — NHTSA manufacturer communication 10113930, physical PDF page 1.