The most basic power lock circuits combine a switch, relay, solenoid, fuse and related wiring. You can find and fix problems on the circuit by following the electrical flow along the circuit and checking specific components or wires where electricity seems to stop when it is suppose to keep flowing. Troubleshooting may be done with the use of an ohmmeter and a simple 12V test light.
Remove the lock switch from the door with the inoperative lock.
Set your ohmmeter to the lowest ohm range.
Touch the switch center prong with one of the ohmmeter test leads and the other two prongs with the other test lead as you operate the switch. If your meter reads infinite resistance in either one of the side prongs on the switch, replace the switch. Otherwise, go on to the next step.
Check the switch power wire between the switch and its power source toward the driver's door or the fuse panel and look for a possible short or open.
Remove the door trim from the door with the nonfunctional lock switch.
Hook the wire clip from your test light to the door metal structure.
Back probe the power wires on the switch solenoid with the test light as you operate the door switch. If the test light glows as you operate the switch, replace the solenoid. If the test light does not glow, go on to the next step.
Back probe the incoming and outgoing power wires from the lock relay with the test light as you operate the switch. If the test light glows as you back probe the incoming voltage wire but not on the outgoing power wire, replace the relay.
Check the power-lock circuit fuse and make sure the fuse is not blown. The fuse is located underneath the dashboard on most vehicle models. If the fuse is in good condition, go on to the next step.
Operate the lock switch on the driver's door and listen for a click sound coming from the relay. If there is no sound, check the door switch for continuity as described on the first section. Replace the switch if necessary.
Remove the door trim and check for incoming voltage at the switch. Clip the test-light wire to the door metal structure and touch the switch power-wire with the tip of the test light. If the test light does not glow, check the power wire between the switch and the fuse panel, looking for shorts or opens.
Troubleshoot the power lock relay and solenoid with the test light as described in the previous section and replace components as necessary.
