What does the code P1633 mean - 2002 Mercury Sable

P1633 Keep Alive Power Voltage Too Low (Ford/Lincoln/Mercury) It means that at sometime in the past the battery voltage was below specs at the PCM and set the code.The battery provides 12 volts to power everything,the car probably sat for a while and probably took a long time to start or had to be jumped off to start.... when the voltage got below 9 volts it set the code. The code is more of a indicator for info than anything.. the computer learns as you drive and all the code is saying is that it lost it's power to itself and can loose that learned information. Have a shop inspect the battery for being weak or dying as well as your battery connections.. if all is fine, I wouldn't worry. You can have the error reset by a mechanic or auto parts store. If you are paitent enough, the computer will reset itself over time.