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In my Bathroom, the light switch is located on the left hand wall. The duplex outlet is on the forward wall. On each side of The forward wall (left & right) are the fixtures, one each. When I flip the light switch to on, only the left hand side light fixture turns on. I've tried new bulbs, even took the front of the fixture off and all looks well. I have a digital multimeter and tools. How do I properly diagnose and fix this issue?

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  • Has it ever worked? If so, what changed or happened when it stopped working? Mar 8 '16 at 12:12
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From your description, it appears that you have 1 switch that controls one duplex outlet and 2 lights that are plugged into those outlet receptacles. If my interpretation is correct, then do the following:

  1. Reverse the light plugs at the outlet receptacles & test to see if the lights illuminate. If only the left light illuminates, then both of your outlet receptacles are working properly & the fault is in the right plug/wiring/fixture/etc. If the left light does not illuminate & the right light does illuminate, then one of your outlet receptacles is bad. If neither light illuminates, then you probably have multiple problems to troubleshoot.

  2. Assuming you are in the US: Use this video procedure to properly test your outlet power. Note: If your multimeter is not auto-ranging, then you would manually set your multimeter to AC mode & your voltage scale to 200V before testing your 120V outlets.

  3. To test an incandescent bulb or the wiring to your light fixture, you would first disconnect power to your light & then use the Ohms mode of your multimeter. Click here to see a video of generally how to do this. You are essentially searching for an open circuit. That means electric power cannot pass through the plug or wiring or contacts or bulb filament or through some other part of the light circuit.

  4. In your situation, the most common fix actions are: replace a bulb, clean corrosion off a metal contact point/s or switch, and/or fix a loose/disconnected/broken wire. If you have florescent lights, then you may need to replace a starter (condenser) or a ballast.

hth, best regards!

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Turn the power off and take a tiny flathead screw driver. Pry the center prong in the socket so it is not flat, nor vertical, but close to an angle.

Oftentimes the power is good but the center tab in the socket is jammed flat and cannot make contact with the bulb.

If that is not the problem then Ohm out each wire starting at the socket, to the fixture, to the box, to the other light, and so on and so on. The setting on the digital multimeter is the audible Ohm setting.

Basically Ohms test for continuity and resistance. We want continuity in this case which indicates a non-break in the wire. So for example, to test continuity on the neutral you would test both neutrals in the working light to the one neutral in the non-working light. And the same for the black wires. Also, the center tab in the socket ( the hot ) should be tested for continuity to the fixture wires, as well as the outer threaded portion of the socket ( the neutral ).

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  • Using Volts on the center of the lamp socket to the shell with power on will tell a better story if you have power. Also verify the bulbs do work, I have had new lamps fail when they had been on the shelf for a long time. Use ohms to test the bulb, or just try it in the working fixture. other than that the center conductor pry up as @Kris suggests is probably the problem.
    – Ed Beal
    Mar 8 '16 at 14:18
  • According to a strict interpretation of the question in the title, OP should be advised to pry up the socket center button with one of the probes from his multimeter instead of with a tiny screwdriver. Mar 8 '16 at 20:14
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First , use your multimeter in voltage mode to see if power is even getting to the right-hand lamp. This tells you if the problem is in the fixture or the wiring.

If you see no voltage between the lamp hot and neutral, check the hot side and the neutral side independently. If your duplex outlet is working then you can insert one of your meter probes into its wide slot (not the round ground hole) for a neutral node reference. Voltage between the lamp hot and an independent neutral means the lamp neutral supply is broken somewhere. No voltage means the lamp hot supply is broken.

If it's the wiring then jiggle and tighten everything:

Turn off the power at the circuit breaker. Remove the covers from all four junction boxes and tighten every connection you can find.

If the fault is a loose connection, the most likely location is at the other end of the cable to the right-hand lamp. This is probably at the outlet or at the left-hand lamp.

If that doesn't work then you must "trace the electrical fault":

With the power on, very carefully use your multimeter to trace the voltage from one connection to the next. Unfortunately you have to simultaneously learn how the circuit is supposed to work, and where the fault is.

You need to figure out where power comes into the room. Don't assume it's at the switch. The outlet box is the more likely candidate.

If you're still stuck, which is highly unlikely, you have to take it apart to fix it:

Turn off the power at the circuit breaker. Thoroughly document every connection. Put tags on the wires if necessary. Disconnect everything.

Carefully rebuild the circuit one cable at a time, Use your multimeter to trace the power through each connection as you make it. By "carefully" I mean you have to turn off the power to make each connection, then turn on the power to test it.

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  • If you have any difficulty figuring out the power flow in the circuit, describe, draw, or photograph what you find in each of the four boxes. There are at least three people here eager to look over your shoulder and give advice. Mar 8 '16 at 5:08
  • Using the outlet for the neutral conductor may overlook a broken neutral in the light. the voltage should be measured center to shell in voltage mode.
    – Ed Beal
    Mar 8 '16 at 14:22
  • @Ed Beal: A broken neutral is one of the things I'm looking for. No voltage between lamp hot and neutral = broken hot or broken neutral - but which? Check for voltage between hot and known good neutral: voltage seen = the lamp neutral is broken, no voltage = lamp hot is broken. I will edit my answer for clarity. Mar 8 '16 at 19:52
  • Ok you never said you measured from the center to the shell and saw no voltage. The left side of the 120V receptacle with the ground at the bottom is the neutral , The right side is the hot. With meter on volts AC connect to left of outlet then to center should read 120V if not open hot. With meter on the right side of outlet to the shell (screw part) it should read 120v of not open neutral. but first test should be center to shell in the fixture.
    – Ed Beal
    Mar 8 '16 at 21:48

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