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Make sure the battery cables are ok.

Unplug the O2 heater relay to see if the problem goes away. If it does, check if the Gray/Yellow wire at the relay is grounded all the time. If it is grounded all the time, the PCM is bad or the wire is grounded. If the wire is not grounded, the relay is stuck in the energized position.

The Yellow wire from the ignition switch keeps powered up with the key off, only if the engine is running when the key is turned off. If the engine is not started, power comes and goes with the switch.

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