Lexus 2012 RX450h Owner's Manual page 915

Emergency start function
When the hybrid system does not start, the following steps can be used as
an interim measure to start the hybrid system if the "POWER" switch is
functioning normally:
Set the parking brake.
STEP 1
Shift the shift lever to P.
STEP 2
Turn the "POWER" switch to ACCESSORY mode.
STEP 3
Press and hold the "POWER" switch for about 15 seconds while
STEP 4
depressing the brake pedal firmly.
Even if the hybrid system can be started using the above steps, the system
may be malfunctioning. Have the vehicle inspected by your Lexus dealer.
5-2. Steps to take in an emergency
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