I have a lexus ls400 and the fuel gauge stop

The sending unit located in the gas tank must be replaced on most of these cars you can gain access to it by removing the back seat revealing an access cover allowing you to get to the sending unit remove retaining screws or locking ring and replace. You probably should be grounded and have a Haynse repair manual handy to walk you through this job it's not the easiest job but not to hard either unless the gas tank must be dropped down to access the sending unit if that's the case have a shop do the job that job is pretty tough.