WARNING: This page is about a different car, the 2002 Lexus LX 470 and 2001 Lexus LX 470. However, it is still accessible from the selected car via links, so may be relevant.
For wiring diagram see Figure. For testing procedure see Fig 1 & Fig 2
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Fig 1: Testing Procedure DTC Always On, Malfunction In ECU (1 Of 2)