I have a 2008 ford ranger, the remote to lock and unlock the doors was working,then just stopped working. the central locking still works when I am inside the my truck. Please advice me what to check.

The doors do not lock/unlock using remote keyless entry (RKE) transmitter
RKE transmitter
RKE transmitter battery
RKE transmitter programming
Aftermarket system
High power devices
TV/radio transmission towers
Smart junction box (SJB)
Go To Pinpoint Test D .
Pinpoint Test D: The Doors Do Not Lock/Unlock Using The Remote Keyless Entry (RKE) Transmitter

Normal Operation
Remote locking and unlocking of the doors is accomplished by the smart junction box (SJB) receiving a command message from the remote keyless entry (RKE) transmitter. The SJB processes the command and provides voltage and ground to the door lock actuators.

All RKE transmitters must be present to begin diagnosis of the RKE system.

Aftermarket or dealer-installed systems may adversely effect the RKE system operation. These systems should be disconnected before diagnosing any RKE concerns.

Possible Causes
RKE transmitter battery
RKE transmitter programming
RKE transmitter
aftermarket systems
high power devices
TV/radio transmission towers
SJB

PINPOINT TEST D: THE DOORS DO NOT LOCK/UNLOCK USING THE REMOTE KEYLESS ENTRY (RKE) TRANSMITTER
NOTE: All RKE transmitters must be present to begin diagnosis of the RKE system.

NOTE: Aftermarket or dealer-installed systems may adversely affect the RKE system operation. These systems should be disconnected before diagnosing any RKE concerns.
Try this first .
Remote Keyless Entry Transmitter Programming
CAUTION: All remote keyless entry (RKE) transmitters must be programmed at the same time.

Before entering the program mode, the vehicle must be electronically unlocked with the RKE transmitter or the door lock control switch while the driver door is open.
Turn the ignition switch from OFF to RUN 8 times within 10 seconds, with the eighth turn ending in RUN. If the module successfully enters program mode, it locks and then unlocks all the doors.
Within 20 seconds, press any button on an RKE transmitter and the doors lock and then unlock to confirm that each RKE transmitter is programmed. Repeat this step for each RKE transmitter.
If the door locks do not respond for any RKE transmitter, wait several seconds and press the button again. If the door locks still fail to respond, refer to Inspection and Verification. Make sure that no more than the maximum number of RKE transmitters is being programmed.
Exiting the programming mode is accomplished if 1 of the following occurs:
The key transitions to the OFF position.
20 seconds have passed since entering programming mode or since the last RKE transmitter was programmed.
The maximum number of RKE transmitters have been programmed. The doors do not lock/unlock using remote keyless entry (RKE) transmitter RKE transmitter RKE transmitter battery RKE transmitter programming Aftermarket system High power devices TV/radio transmission towers Smart junction box (SJB) Go To Pinpoint Test D . Pinpoint Test D: The Doors Do Not Lock/Unlock Using The Remote Keyless Entry (RKE) Transmitter Normal Operation Remote locking and unlocking of the doors is accomplished by the smart junction box (SJB) receiving a command message from the remote keyless entry (RKE) transmitter. The SJB processes the command and provides voltage and ground to the door lock actuators. All RKE transmitters must be present to begin diagnosis of the RKE system. Aftermarket or dealer-installed systems may adversely effect the RKE system operation. These systems should be disconnected before diagnosing any RKE concerns. Possible Causes RKE transmitter battery RKE transmitter programming RKE transmitter aftermarket systems high power devices TV/radio transmission towers SJB PINPOINT TEST D: THE DOORS DO NOT LOCK/UNLOCK USING THE REMOTE KEYLESS ENTRY (RKE) TRANSMITTER NOTE: All RKE transmitters must be present to begin diagnosis of the RKE system. NOTE: Aftermarket or dealer-installed systems may adversely affect the RKE system operation. These systems should be disconnected before diagnosing any RKE concerns. Try this first . Remote Keyless Entry Transmitter Programming CAUTION: All remote keyless entry (RKE) transmitters must be programmed at the same time. Before entering the program mode, the vehicle must be electronically unlocked with the RKE transmitter or the door lock control switch while the driver door is open. Turn the ignition switch from OFF to RUN 8 times within 10 seconds, with the eighth turn ending in RUN. If the module successfully enters program mode, it locks and then unlocks all the doors. Within 20 seconds, press any button on an RKE transmitter and the doors lock and then unlock to confirm that each RKE transmitter is programmed. Repeat this step for each RKE transmitter. If the door locks do not respond for any RKE transmitter, wait several seconds and press the button again. If the door locks still fail to respond, refer to Inspection and Verification. Make sure that no more than the maximum number of RKE transmitters is being programmed. Exiting the programming mode is accomplished if 1 of the following occurs: The key transitions to the OFF position. 20 seconds have passed since entering programming mode or since the last RKE transmitter was programmed. The maximum number of RKE transmitters have been programmed.