2005 Cadillac Deville service engine soon

The powertrain control module supplies a ground to turn on the malfunction indicator light(MIL). If it is not coming on in bulb check, it could be the result of a faulty PCM, or fault with circuit wiring or terminals. You have two connectors in the circuit, C101 that is mounted to the left strut tower by the cruise module, and C200 which is under the dash pad, on the left side by the A-pillar. It dead ends at PCM connector C1 pin 55. C1 is the blue 56 pin connector. It comes in the cluster at pin A8 of the green 16 pin connector C2. The color of the circuit is brown with white trace. It is a relatively simple circuit, it is the connectors and terminals that add complexity. The PCM connector houses a gem that is called the Micro.64mm terminal. These terminals are problematic, and if you use the wrong tool to check them, you will destroy it. If you have loose terminal tension on any terminal in the circuit, it could give you an open circuit. If you choose to try and narrow it down, you can check continuity from PCM C1 pin 55 to C101 pin C2 on the engine harness side. If you have continuity, then check from C101 pin C2 body harness side to either C200 pin A6, or IPC C2 pin A8. If you have continuity there, ground the wire by backprobing the PCM connector, and turn key on. If the MIL lights, you either have a faulty PCM, or Micro.64 terminal in the PCM connector.