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Glow Plugs: Igniting Efficiency for Engine Excellence

By R&D
Published on April 24th, 2024

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Glow Plugs heat fuel as it is injected into a diesel engine’s combustion chambers during a cold start to promote combustion and remain operating for a short time afterward to reduce emissions.

Glow Plugs


Glow plugs are located in the cylinder head, one for each cylinder. Glow plugs can be recognized by the electrical wire connected at the top terminal by a small nut or rubber boot.

In some vehicles, including certain Volkswagen TDI models, glow plugs are located beneath a tubular rubber harness. On Ford 7.3 liter Power Stroke diesel engines, the glow plugs are inside the valve covers. (Cummins diesels in many Dodge light trucks do not employ glow plugs; instead, these vehicles use a plenum heater grid.)


Diesel engines operate by then injecting fuel air which has been highly compressed by the upward travel of a piston in its cylinder. Compression heats the air to about 1,600º F., so injected fuel combusts without an external source of ignition, such as a spark plug. However, when either the diesel engine or the fuel is below operating temperature, the air temperature may not be increased sufficiently for compression ignition

The glow plug solves that problem. The underlying theory is the same used in an ordinary kitchen toaster: electrical current routed through a thin wire heats the wire. In glow plugs, however, the wire is arranged as a coil and gets much hotter than a toaster: typical glow plugs heat to about 1,800º F. and advanced design glow plugs can reach 2,500º F.

In a glow plug, the coil of thin wire is either housed in a metal “glow tube” filled with magnesium oxide powder or is encased in a hard ceramic casing of silicone nitrite. Both materials freely transmit heat generated by the coil.

The glow tube or ceramic casing is sealed into a steel “body” or “shell” which is threaded to screw into the cylinder head. The top of the wire coil is connected, inside the glow plug body, to an “electrode” shaft that is also sealed into the body and projects as a threaded end from the top of the body. The wire carrying current to the glow plug attaches to this “terminal.”

“Self-regulating glow plugs” divide the coil into two connected elements, one above the other, in which resistance of the upper ‘regulating coil’ increases as temperature increases and thereby reduces current flow to the ‘heating coil’ at the plug’s tip, preventing overheating the glow plug. Some older diesel vehicles, including Mercedes-Benz automobiles manufactured in the 1970’s, employ glow plugs with a loop extending from the body, rather than a glow tube.

Glow Plugs

When the key is turned to start a cold diesel engine, battery voltage is supplied to the glow plugs, which reach operating temperature within 2 to 5 seconds. Once the engine has started, glow plugs remain on for a short time, termed “after glow.”

The duration of after-glow depends on engine temperature and engine design. In older diesels, after-glow typically does not exceed two minutes. In modern diesels, after-glow may continue for five minutes. The glow plugs are then turned off, either by the “glow plug controller” (GPC) by the “glow plug control module” (GPCM) based on engine oil temperature, time duration, and other sensor inputs. The GPCM also measures glow plug amperage, a measure of the plug’s current draw.

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Drivability Symptoms

The most obvious symptoms of glow plug malfunction are hard starting (particularly in cold weather), rough idle or shuddering during warm-up, and white exhaust smoke from start-up until the engine has warmed.

The instrument panel “wait to start” light may remain illuminated longer than normal. However, all of these symptoms can have other causes, including electrical faults in the circuits supplying current to the glow plugs, the “glow plug relay” and GPC, or the GPCM. On 1997 and later diesel automobiles and light trucks, malfunction of a glow plug illuminates the instrument panel malfunction indicator light (MIL).

Inspection, Test, and Diagnosis

1997 and later diesel automobiles and light trucks are equipped with Second Generation On-Board Diagnostics (OBD2) that illuminates the Malfunction Indicator Lamp (MIL) and stores a Diagnostic Trouble Code (DTC) when a malfunction occurs that could increase emissions by 50% or more above federal limits.

On these vehicles, DTC P0380 is set when the Engine or Powertrain Control Module (ECM/PCM) determines that one or more glow plugs have substantially greater resistance than others, even though all are within specifications.

Later model OBD2 diesel vehicles also can set DTCs P0671 through P0682, which are cylinder specific codes indicating an open or short in a specific glow plug circuit for cylinders 1 through 12, respectively. All of these codes can be set by a defective glow plug, but may have other causes including defective wiring, a defective glow plug relay or GPC, or defective GPCM.

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If one of these codes has been set, measure electrical resistance of each glow plug with a digital multimeter (DMM). Connect the black lead to the COM port and the red lead to the ohms (Ω) port. (“Ohms” is the scale measuring resistance.)

With the engine off, disconnect the wire from the glow plug terminal. Touch the positive lead of the DMM to the terminal and the negative lead to a good ground, such as the cylinder head. The displayed reading is the glow plug’s resistance.

Test each glow plug. General Motors Duramax diesel glow plugs should have less than 1 ohm resistance. (Access Duramax glow plugs by removing the inner fenders.) Ford Power Stroke diesel glow plug resistance should be between 0.6 and 2.0 ohms. (Power Stroke glow plugs can be tested by removing the pigtail from the connector on the inboard side of the valve covers. The two pins furthest forward and two pins furthest rearward on the connector are for the glow plugs.)

A failed glow plug should be inspected for indications of engine malfunction. If the glow plug tip has been eaten away, the fuel injector spray pattern is incorrect. A bulging glow tube tip indicates excessive voltage, usually caused by a defective glow plug controller.


Replacement of glow plugs does not require reprogramming.


Auto mechanic replacing glow plugs in car.

To minimize the risk of breaking a glow plug during removal (which would drop fragments into the combustion chamber), apply penetrating oil and allow it to soak into the glow plug threads before attempting to remove the glow plugs.

Glow plugs should be removed with the engine as close to operating temperature as possible, consistent with personal safety. Disconnect the electrical wire from the terminal and use a deep well socket to unscrew the glow plug. The socket must be firmly and squarely seated to avoid side loading that could break the plug.

As a rule, not more than 15 ft. lbs. of torque should be applied to remove an 8 mm glow plug, 25 ft. lbs. to remove a 10 mm glow plug, and 35 ft. lbs. to remove a 12 mm plug. If the tip of the glow tube has swollen and will no longer fit through the threaded opening in the cylinder head, a “glow plug extractor,” resembling a nut cut into two halves, can be used.

The extractor is clamped around the exposed threads of the glow plug body, effectively extending the threaded hole upward and allowing continued unscrewing to squeeze the plug tip through the hole without breaking it off.

When DTC P0380 has been set by a glow plug, all glow plugs should be replaced so that all glow plugs will have the same resistance. Replacement glow plugs should be of the same specification and length as the original equipment glow plugs, and should cross-reference to the original equipment glow plug part number.

Before installing new glow plugs, the threads in the cylinder head should be reamed using a “glow plug reamer” with a light grease coating to remove accumulated carbon deposits. Resistance of the new glow plugs should be verified with a DMM before installation.

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