If the remote lock/unlock is intermittent and not the battery, the sensor may need to be cleaned. To clean the sensor, use a cotton swab dipped in rubbing alcohol. Gently rub the swab around the sensor until it is clean.
Here are some additional tips for troubleshooting the remote lock/unlock system on a 1999 RX 300:
* Check the key fob battery. If the battery is low, the remote will not work properly.
* Try reprogramming the key fob. Instructions on how to do this are in the owner's manual.
* Check the fuses for the remote lock/unlock system. The fuses are located in the fuse box under the hood.
* If the remote lock/unlock system still does not work, you may need to take the vehicle to a dealer or qualified repair shop.