POA fail 1 - 1990 Jaguar XJ6

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The sequence of switch closures received by the BCM

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The status of the engine run flag.

Important: Any ignition switch states not covered in this table indicates a DTC setting situation.
The chart indicates the modes detected and transmitted by the BCM:
Power Mode Selected/Expected
Engine Run Flag Serial Data
Sampled Ignition Signal State
Power Mode State
Accessory
Ignition 0
Ignition 1
Crank
OFF-LOCK
0
0
0
0
0
OFF
Accessory
0
1
0
0
0
ACCESSORY
RAP
0
1 to 0
0
0
0
RAP
OFF-UNLOCK
0
0
1
0
0
UNLOCK
RAP
0
1 to 0
1
0
0
RAP-UNLOCK
Run
0
1
1
1
0
RUN
Crank
0
0
1
1
1
CRANK
Crank
1
0
1
1
1
CRANK
Run
1
1
1
1
0
RUN
Fail-safe Operation
Since the operation of the vehicle systems depends on the power mode, there is a fail-safe plan in place should the BCM fail to send a power mode message. The fail-safe plan covers modules with discrete ignition signal inputs as well as those modules using exclusively serial data control of power mode.
Serial Data Messages The modules that depend exclusively on serial data messages for power modes stay in the state dictated by the last valid BCM message until they can check for the engine run flag status on the serial data circuits. If the BCM fails, the modules monitor the serial data circuit for the engine run flag serial data. If the engine run flag serial data is True, indicating that the engine is running, the modules fail-safe to "RUN". In this state the modules and their subsystems can support all operator requirements. If the engine run flag serial data is False, indicating that the engine is not running, the modules fail-safe to "OFF-AWAKE". In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.
Discrete Ignition Signals Those modules that have discrete ignition signal inputs also remain in the state dictated by the last valid BCM message received on the serial data circuits. They then check the state of their discrete ignition input to determine the current valid state. If the discrete ignition input is active, B+, the modules will fail-safe to the "RUN" power mode. If the discrete ignition input is not active, open or 0 voltage, the modules will fail-safe to "OFF-AWAKE". In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.
You probably won't understand any of this info. your best but would be to take it to a qualified repair shop that has proper diagnostic test equipment !