There could be several reasons why the sub tank in your Land Cruiser is not working. Here are a few possible causes and troubleshooting steps:

1. Check the fuel transfer switch: The Land Cruiser typically has a switch that allows you to select between the main fuel tank and the sub tank. Ensure that the switch is set to the sub tank position. Sometimes, this switch can get stuck or become faulty, preventing fuel from being drawn from the sub tank.

2. Check the fuel lines and connections: Inspect the fuel lines and connections leading to the sub tank. Look for any signs of leaks, loose connections, or blockages. If you find any issues, they will need to be repaired or replaced.

3. Check the fuel pump: The sub tank has its own fuel pump that supplies fuel to the engine. If the pump is not functioning properly, fuel may not be transferred. You can check if the fuel pump is operating by listening for a humming sound when the ignition is turned on. If you do not hear any noise, it may indicate a faulty fuel pump that needs to be replaced.

4. Inspect the fuel level sensor: The sub tank is equipped with a fuel level sensor that sends signals to the dashboard gauge. If the sensor is malfunctioning, it could lead to inaccurate readings or prevent the sub tank from being used. You can use a multimeter to test the resistance of the sensor to determine if it is working correctly.

5. Check for blockages in the sub tank: Over time, debris or sediment can accumulate in the sub tank, causing blockages. This can prevent fuel from flowing freely. You may need to remove the sub tank and clean it or have it professionally cleaned to ensure proper operation.

If you have gone through these troubleshooting steps and still cannot resolve the issue, it is recommended to consult with a professional mechanic who can diagnose the problem accurately and suggest the appropriate repair or replacement.