No. A diesel ignites by compression - the glow plug only warms the chamber so compression is enough when the engine is cold.

Compressing air raises its temperature, and in a diesel that temperature is what ignites the injected fuel.

In a cold engine, heat leaks into cold metal faster than compression can build it, so ignition is unreliable or fails.

The glow plug adds heat locally, raising the chamber temperature enough for compression ignition to work.

Once the engine is warm they are not needed for running at all, which is the opposite of a spark plug's continuous role.

That difference matters diagnostically: a warm diesel that runs badly does not have a glow plug problem.

Running the plugs after the engine has started, to reduce white smoke, noise and unburnt hydrocarbons while it warms up.

A cold diesel burns poorly for the first minutes, producing white smoke, a harsh combustion noise and high emissions.

Continuing to heat the chamber after starting improves that combustion substantially.

On many modern vehicles post-heating runs for several minutes, controlled by the engine management according to temperature.

It means the plugs are working far more than the pre-heat time suggests, and their duty is correspondingly higher.

It also explains why a failed plug can present as rough cold running and smoke rather than as difficulty starting.

By resistance measurement to find open-circuit plugs, and a current clamp to see which are actually drawing - after testing the relay.

A healthy glow plug has a very low resistance, and an open-circuit one reads infinite and is conclusively failed.

Resistance alone can be misleading, so measuring the current each plug draws during the heating cycle is the better test.

The control relay or module supplies all of them, so a single failure there stops every plug and looks like total failure.

Test the supply at the plugs during a pre-heat cycle before condemning anything, since a relay is cheaper and far easier to change.

Fault codes on modern vehicles frequently identify the individual circuit, which narrows it immediately.

They seize into a hot, carbon-rich part of the head, and the body is relatively slender - so removal torque can exceed what the plug will take.

Carbon builds up around the plug body where it enters the chamber, effectively welding it in place.

Corrosion in the threads adds to it, particularly on plugs that have been in place for many years.

The plug's tip is thinner than its threaded body, so it fails at the transition and leaves the tip in the head.

Extracting the remains needs specialist tools and, in bad cases, removal of the cylinder head.

Warming the engine to a moderate temperature, soaking with penetrating oil, and working the plug back and forth in small increments all reduce the risk considerably.

There is a real case for it - the risk of breakage rises sharply with age, and the consequence is out of all proportion to the part cost.

A glow plug is inexpensive; extracting a snapped one can require the cylinder head to be removed.

The longer a plug stays in place, the more carbon and corrosion accumulate and the more likely it is to break.

Replacing them as a set at a sensible interval, rather than individually when they fail, keeps the removal risk manageable.

It also means the job happens in a workshop by choice rather than on a cold morning with a vehicle that will not start.

For a fleet running diesels in cold climates, this is one of the better preventive maintenance decisions available.

Yes - they have the same age and heat exposure, and the labour is nearly all in the access.

When one plug has failed, the others have endured identical duty and are close behind.

Access is usually the whole job on a modern engine, so replacing one and returning for the next is expensive.

It also avoids repeating the breakage risk on plugs that have aged further in the meantime.

Fit the correct type: modern systems use ceramic and metal plugs with different voltage and heating characteristics, and they are not interchangeable.

Torque to specification - these are slender components in an aluminium head and are easily damaged by over-tightening.

A warm one will start and run normally. A cold one may not start at all, and if it does it will run roughly with heavy white smoke.

Because they are only needed for cold starting, a failure often goes unnoticed through warm weather.

The first cold morning is when it becomes apparent, which is why glow plug faults cluster seasonally.

Starting a cold diesel repeatedly without them puts heavy load on the battery and the starter, and washes fuel past the rings.

White smoke from a cold engine is unburnt fuel, and it is both an emissions problem and a sign of fuel diluting the oil.

Using aerosol starting fluid as a substitute is a bad idea on an engine with glow plugs, and can cause serious mechanical damage.