Twas the Day Before Christmas and In a Few Blinks, Not An LED Christmas Light was Burning, those Darn Little Finks.

Well, it wasn’t that bad, but a strange thing happened this year. Several Strange Things.

We have LED Lights and we lose a Set or more Per Year. Incandescent Sets, we had those for years before they became an issue. Remember, LED’s last Forever – Bwhahahahahahahaha.

So, we have an LED set that looks like they’re C9 Bulbs, and they go around the top of the Porch. Most of the Blue LEDs burned-out, and it wasn’t really a problem, really just a few were out, but then the set went out. Ugh.

My wife was mad, and I set about to see if I could correct the situation.

This Blog Article Got Me Started:

Fixing Led String Lights

And I was using a Small, Pocket Digital Volt Meter and wasn’t getting the results I expected, so I got my Fluke 12. To avoid confusion I’ll avoid a Dissertation on the Meters, save a few points.

1. They Both Have a Diode Checking Setting.

2. The Pocket Digital Volt Meter with AA Batteries produces 1.5 Volts for Checking Diodes, the Fluke with a 9 Volt Battery produces 2.7 Volts for Checking Diodes.

The Fluke Link Below is for Testing Diodes and not necessarily fully applicable to Testing LEDs.

Fluke: How to Test Diodes with a Digital Multimeter

Diodes only let current flow in one direction. They are used in Pairs (or more) to Rectify the Incoming AC Power (for Electronics that plug into AC) and make DC Power (with Capacitors used to smooth it out) for Electronics to operate.

Below is a site (and a relevant article) that offers items for Experimental and Learning Purposes and I have purchased and reviewed their meter in a previous Blog, looks like an interesting site:

What is Polarity?

LEDs are Light Emitting Diodes, so they share principles with Diodes insofar as the current can, normally, only flow in one Direction. Using the Fluke 12 (or any Meter that has sufficient Output Voltage on the Diode Test Range), when I connect the Black Lead to one side of the LED, and the Red Lead to the other, if the Diode works, the Meter will show No Reading for one Connection and the Forward Bias Voltage if I Reverse the Lead Connections (Diode Stuff, the Voltage needs to be more than a certain level, very low level, to make the Diode Conduct).

But my use of the Meter was much simpler than that. I found that when the Meter Leads are Connected One Way, No Reading occurs on the Meter, and when I Switch the Meter Leads, if the LED is good, it Lights.

Meter and LED - 1

How about that. So all I had to do was look for 3 Scenarios (Note: No Change in Meter Display means it read the same when the leads were not connected and when they were connected):


For a Good LED Using my Fluke 12:

  • Red Lead at Test Lead #1 Position, and the Black Lead at Test Lead #2 Position, the LED Lights.
  • Black Lead at Test Lead #1 Position, and the Red Lead at Test Lead #2 Position, the Meter Display Doesn’t change, LED Doesn’t Light.

For a Non-Lighting LED that Isn’t Likely to be Stopping the Light String or part of it from Lighting.

  • Red Lead at Test Lead #1 Position and Black Lead at Test Lead #2 Position, the Meter Display Indicates a Low Value, the LED Doesn’t Light.
  • Black Lead at Test Lead #1 Position, and the Red Lead at Test Lead #2 Position, the Meter Display Indicates a Low Value, the LED Doesn’t Light.

In other words, the Meter Display Indicates a Low Value for Both Connections and the LED Doesn’t Light (using my Meter, but be sure your Meter will Light a Good LED before assuming this), indicating a Shorted LED but the Voltage should pass through to keep the remainder of the Lights Lit.


For a Bad LED that might be Stopping the Set from Working.

  • Red Lead at Test Lead #1 Position and Black Lead at Test Lead #2 Position, the Meter Display Doesn’t change, LED Doesn’t Light.
  • Black Lead at Test Lead #1 Position, and the Red Lead at Test Lead #2 Position, the Meter Display Doesn’t change, LED Doesn’t Light.

In other words, the Meter Display Doesn’t Change for Both Connections and the LED Doesn’t Light, indicating an Open LED Blocking the Voltage from passing through and keeping the remaining LEDs from Lighting.


If you made it this far, you don’t need a Fluke 12 to test LED Lights, they sell rather inexpensive Testers for LED Christmas Lights (be sure anything you select is Intended for use with LEDs or will also Test LEDs). Search online for that and read reviews. It will likely function like my Fluke 12 did, one way, nothing, the other way, it Lights, but be much simpler to understand and usually just requires placing the Diode Against Contacts on the Tester.

Back to the LEDs though. We noticed that the Blue LEDs only were going out. Over time, one, then another, and so on. Eventually the Set Went Out. When I decided to try and fix the set, I pulled one LED at a time, all 25 LEDs in a Set of 25, regardless of the color. I found that the Blue ones were indeed not lighting, but the Fluke Article Above says that Diodes either Short or Open. If they Short, in the case of LED Christmas Lights, the rest of the set remains lit. But if they open, there goes that set, or half the set, depending on how the set is wired. The LEDs that weren’t Lit showed (all but one) a Meter Reading the same (a Low Voltage Level) when the leads were connected either way (LED is Shorted). But one had no change on the Meter Display (i.e. it read the same when the leads were not connected and when they were connected) regardless of how the leads were connected (LED is Open), and when I tried a temporary repair, the set came on. We ordered Replacement LEDs, already in the Holders needed for the sockets our LED lights used, and when they came, replaced the Blue LEDs that were not lighting and one that was open, and voila, the set lit.

But half the Deck Lights went out after that (the First Set to Fail were on the Porch). Hmmm? Guess what, the Blue Lights had been going out on the Deck, just like the porch, and instead of Testing all the LEDs, I just did the Blue LEDs. Lo and Behold, most of the Blue LEDs didn’t light when tested, and one was found to be Open. I bought 80 Replacement LEDs, in a selection of Colors, but used all the Blue ones on the Porch. On the Deck, I started replacing the Blue ones with Yellow, there is a Blue Cap over them, but they weren’t bright enough. I noticed that Green seemed to work similarly well to the Blue LED Replacements for LEDs with a Blue Cover.

Porch Lights, we have 2 Sets of 25 LED Lights connected together. 25 C9 Size LED Covers per Set. The Set that had gone out was 25 LEDs Long.

Deck, we have 1 Set of 50 LED Lights. 50 C7 Size LED Covers per Set. 25 LEDs were no longer working, even though they shared the same cord, all 25 Out were on the Far End of the Set from the plug.

IMG_20211219_134008 - 1

Both the 25 LED and 50 LED Sets have 2 wires from the Power Cord, that become 3 Wires at the first Light Socket. These LED Light Sets use Resistors and a Diode in a small capped socket like compartment early on the set on the Plug End. These must rectify the Power to DC, and Reduce the Current. The 3 Wires, in part, were needed to carry 120V AC to the Connector on the Far End of the set to power additional lights plugged into it.

WITH THE LIGHT SET UNPLUGGED (Caution, Shock, Shorts, and Electrocution can occur Despite these being LED Light).

If the set comes with Spare LEDs mounted in Inserts for the Socket, you can tell they intended the user to change bulbs no longer working. Consult the original Box, Instructions, or find the Set Online if you are unsure.

On my Sets, C-9 or C-7 Bulb Shaped Colored Caps Pull Out of recess mounted around the LED Socket, see Below. Once you learn how to do it, it becomes much easier. Hold the Base Securely, but not by the wires. Pull Up and to the Side Slightly on the C-7 or C-9 Cover.

Once the Cover is off, Using a Narrow Bladed Screwdriver, if you have Socketed LED Inserts as shown below, pry the LED and its Base from the Socket.

Test the LED as Described above, using an LED Christmas Light Tester, an LED Tester from other sources, or a Digital Volt Meter with a Diode Test Position where the output voltage is high enough to Light the LED. Keep the Meter Connected to the LED for 10 seconds or more, I have on LED that seems to work but goes to a Shorted Condition after a limited time. This would not stop the set from working and if you missed this, the Bulb will go out in use, just replace it again.

If using a Suitable Meter, Hold the Black Lead on the Shorter of the 2 LED Leads as seen in the Holder on the Left. Hold the Red Lead on the Longer of the 2 LED Leads as seen in the Holder on the Left. The LED Should Light if it is good.


If the LED doesn’t Light, take notice of the Meter Display and then reverse the Meter Leads, Hold the Black Lead on the Longer of the 2 LED Leads as seen in the Holder on the Left. Hold the Red Lead on the Sorter of the 2 LED Leads as seen in the Holder on the Left.

If they both do not change the Meter Display (in Diode Mode) from the state the Display Indicated before Connecting the Test Leads vs the Display showing the Test Leads Connected Either Way, the LED is open and needs replaced.


If they both change the Meter Display (in Diode Mode) to a Display showing a Low Value with the Test Leads Connected Either Way, the LED is Shorted and although it probably is not the reason the Set is Lighting, it needs replaced to have the LED Work.

IMG_20211219_133905 - 1

When Inserting New LEDs Mounted in their Socket Inserts, watch for the Polarity Mark to Align with the Socket. Halfway through replacing the LEDs and Socket Inserts, I realized they were no longer Marking the Insert for Polarity. All is not Lost. Look at the LED and Socket Insert to the Left, above. One Connection is much longer than the other. Correspondingly the Sockets have a connection deeper down into the socket and one that is nearer the top. Simply Match the Socket Insert with the Socket so the Longer LED Lead will contact the Deeper Socket Connection.

Make sure the Leads are Centered on the Socket Insert and push to the center if necessary. Make sure they are connected correctly and Inserted to have the LED Leads Contact the Socket Connection Points.


Author: Dr-Artaud

A Doctor that is not a Doctor, but named after a character in the movie "No Such Thing", as is the Avatar.

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