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Is your diesel-powered vehicle difficult to start when cold? If so, there may be a problem with the glow plugs.

Of course, because many different issues can cause hard starting, you’ll need to do some troubleshooting to determine whether the glow plugs are to blame. The good news is, you can perform a simple glow plug resistance test at home with a digital multimeter (DMM).

automotive glow plugs
The glow plugs act as heating elements to preheat the air in the combustion chambers, making the vehicle easier to start.

What is a Glow Plug?

Gas-powered engines use spark plugs to ignite the air-fuel mixture inside the combustion chambers. On the other hand, Diesel engines rely on the heat from the compression of air to ignite fuel. Because of this, diesel engines can be difficult to start when cold.

Many diesel-powered vehicles use glow plugs to address this issue. The glow plugs act as heating elements to preheat the air in the combustion chambers, making the vehicle easier to start. Diesel engines typically have one glow plug per cylinder.

Each glow plug has a terminal, a threaded body, and a tip the protrudes into the engine’s combustion chamber. Applying voltage to the terminal end of the plug allows electrical current to pass through a heating element in the plug’s tip. The plug’s body, which is threaded into the cylinder head, acts as an electrical ground for the heating element.

Modern vehicles have a control module that manages the glow plugs. The module looks at inputs from various sensors, such as the coolant temperature and oil temperature sensors, when determining when to turn on the glow plugs. On many vehicles, the control module activates the glow plugs via a relay.

How to Test Glow Plugs in an Engine

Over time, glow plugs wear out, and their internal resistance changes. When this happens, the plugs may cause hard starting and other issues. Fortunately, testing the glow plugs to see if they require replacement is a fairly straightforward affair.

Note: The following are general guidelines for educational and entertainment purposes only. Consult your vehicle’s factory information for specific repair instructions and recommended safety procedures.

mechanic checking DTC
When the device detects a problem with one or more of the glow plug circuits, it will turn on the check engine light and store a diagnostic trouble code in its memory.

Perform a Preliminary Inspection

If you’re diagnosing the glow plug system on a modern vehicle and the check engine light is on, the first thing you’ll want to do is check for diagnostic trouble codes (DTCs). A control module monitors the glow plug system. When the device detects a problem with one or more of the glow plug circuits, it will turn on the check engine light and store a diagnostic trouble code in its memory.

Note: If you have a scanner with OEM enhanced data, there is also a “key on engine running (KOER)” glow plug test you can perform on select Ford vehicles.

Next, you’ll want to perform a visual inspection of the glow plugs. Look for issues, such as damaged wires and poor connections. Make sure that the electrical connectors going to the glow plugs are clean and tight.

Repair any issues found during the visual inspection, clear the DTCs, and see whether the problem returns.

From there, you can move on to testing the glow plugs as necessary.

Test the Glow Plugs with an Infrared Thermometer

Before you whip out your digital multimeter, you might want to use an infrared thermometer to check for inoperative glow plugs. To do this, first, make sure the engine is cold. Then, cycle the ignition a few times to heat up the glow plugs, and point the thermometer at each plug.

If you find a plug that has a much lower temperature than the others, that plug isn’t working properly, either due to an internal failure or a damaged electrical connector.

Note: This test is only used to locate plugs that aren’t working at all. A glow plug may pass this test and still have carbon buildup or be out of specification resistance-wise.

How to Test Glow Plugs with a Multimeter

glow plug module
You can test the resistance of the glow plugs with a digital multimeter (DMM)

As was mentioned, the internal resistance of the glow plugs changes as the plugs begin to degrade. You can test the resistance of the plugs with a digital multimeter (DMM).

The process for testing glow plugs may vary depending on the type of vehicle you have. For example, if the plugs are difficult to access, you may want to check the resistance at the wiring harness rather than at the plugs themselves. It’s a good idea to consult a repair manual or repair database for the proper test procedure.

In general, however, you can test the resistance of a glow plug by doing the following:

  1. Make sure the vehicle’s ignition is in the OFF position.
  2. Disconnect the electrical connector from one of the glow plugs.
  3. Set your digital multimeter (DMM) to the ohms setting.
  4. Connect the red meter lead to the glow plug’s terminal.
  5. Connect the black meter lead to a good ground, such as the engine block.
  6. Note the resistance reading on your meter. If the meter reads out of limits (OL), that indicates an open circuit and a faulty glow plug. Likewise, if one glow plug has a significantly different internal resistance than the others, that plug is likely faulty and should be replaced.

    When in doubt, consult a repair manual or repair database for the vehicle manufacturer’s resistance specifications.

Keep in mind that a glow plug can be in good shape electrically and still have excessive carbon buildup. You may want to remove any suspect plugs to check for such a condition.

If you find one or more of your vehicle’s glow plugs to be faulty, check out our extensive selection of direct replacements.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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