What are glow plugs and do they need replacing?

Unlike the fuel/air mixture that powers the engine in a petrol car, which needs a ‘spark’ from a spark plug to ignite it, diesel spontaneously combusts when under high pressure, so requires no spark. It does, however, have glow plugs, and their primary role is to aid the initial starting process by heating up the fuel/air mixture when the engine is cold.

Developed in the 1920s, their design remained largely unchanged until the 1970s when the original external coil technology was superseded by a covered coil, which persists with modern glow plugs today, although what’s under the surface has developed considerably since then.

Unlike spark plugs, which are subject to vehicle manufacturer specified replacement intervals, glow plugs generally have no such stipulation, so how do you know when they should be replaced? 

As with any vehicle component, the glow plug has a lifespan, which would broadly be defined by the number of heat cycles it can go through before it starts losing performance. This is very difficult to correlate with the distance a vehicle travels annually, because it is really associated with the number of engine starts, (and latterly diesel after treatment cycles), the glow plugs have to activate, rather than the distance the vehicle has travelled. 

What is for certain is that recent, post-pandemic driving habits of fewer journeys and of a reduced distance, have had a disproportionate impact on diesel cars over their petrol cousins, and this is largely because of the time a diesel engine takes to reach its correct operating temperature. 

As it is fundamentally the temperature of the oil that accurately defines the temperature of the engine, and the oil can often take 10 miles or more of driving to reach its optimal temperature, this has big implications on the performance of the engine, including the glow plugs. So, the more cold starts they make without then reaching their designed engine operating temperature, the faster they will wear out.

Making matters worse, the glow plug now has a role in the diesel particulate filter regeneration process, which is to temporarily raise the temperature of the exhaust gases so they can burn off the soot captured in the filter. This means they have to work extra hard on vehicles that only travel short distances in order to activate the process that would happen naturally during the course of its use in a vehicle that travelled longer distances. 

In short, although vehicles are typically being used less, the glow plugs are having to work harder, which means they need to be replaced more frequently. 

As a result, warning signs of potential glow plug problems range from having trouble starting the engine from cold, to an inability to successfully regenerate the particulate filter. So, despite being a relatively small component, glow plugs have a big influence on the performance of the engine. 

Additional evidence concerning glow plug problems, or their complete failure, can also be identified through diagnostic checks during exhaust emission testing as part of an MOT test, for example, as well as warnings on the vehicle’s dashboard.

• An engine warning light is the first indicator that there could be a fault in the glow plug or heat temperature sensor 
• Difficulty in starting the engine, especially in cold conditions, is an indication that the glow plugs aren’t warming up quickly enough 
• If, when idling, the engine starts to produce white smoke from the exhaust, it could indicate that the glow plugs have burned out. This is especially noticeable under cold start conditions 
• Black smoke coming from the exhaust, or an increase in fuel consumption, can both also be indicators of glow plug faults A problem with your vehicle’s glow plugs is not something that should be tackled yourself, but your local independent workshop will have technicians with the knowledge and understanding to check, and if necessary, replace them.