When a diesel starts struggling to fire up in the morning, the first thing we think of is the glow plugs. And most of the time, that is exactly the problem. At our shop, glow plug replacement is one of the most common jobs as soon as the first cold snap hits.
What Glow Plugs Do on a Diesel
Unlike a gasoline engine that uses spark plugs, a diesel ignites fuel through compression. Air is squeezed so tightly that it heats up enough to ignite the diesel fuel. The problem comes when the engine is cold: the metal of the cylinder and head draws away heat, so compression alone is not enough.
That is where glow plugs step in. They are small heating elements threaded into each combustion chamber, and they warm things up before you crank the starter. When you turn the key to the "on" position, you see the glow plug indicator on the dashboard. Once it goes out, the plugs have done their job and the engine is ready to start.
Symptoms of Faulty Glow Plugs
Spotting the problem is not hard:
- Hard cold starts - the engine cranks longer than usual, especially in the morning or at low temperatures
- White smoke on startup - unburned fuel exits as white smoke during the first few seconds of running
- Rough running until the engine warms up - the engine shakes, knocks, or runs unevenly while cold, then settles down once it reaches temperature
- Check engine light - the ECU detects a fault in the glow plug circuit
- Engine will not start at all at very low temperatures, below minus ten
From our experience, many drivers only notice the issue once temperatures drop below freezing. During summer, the engine starts fine without glow plugs because ambient temperature is high enough. But as soon as cold weather arrives, a missing plug or two makes itself known immediately.
Testing Glow Plugs
We test glow plugs in several ways. The simplest is measuring resistance with a multimeter: a good plug shows low resistance, while a burned-out one shows infinite resistance (open circuit). Another method is measuring the current draw of each plug. With a diagnostic tool, we can also read fault codes related to the glow plug circuit.
Sometimes the problem is the plugs themselves, and sometimes it is the relay or control module that supplies power to them. The relay is responsible for sending current to the plugs, and if it fails, none of the plugs will work. That is actually the easier scenario because replacing a relay is cheaper than replacing all the plugs.
Why Replace Them All at Once
When one glow plug fails, the rest are usually at the same level of wear. If you replace only one, there is a good chance another will fail within a month or two. That is why we always recommend replacing the full set. A single plug costs between 15 and 50 KM depending on the model, and fitting a complete set of four takes an hour or two.
One note: on some engines, glow plugs tend to seize in the cylinder head due to corrosion and heat. When that happens, removal becomes more complicated and requires special tools. If the plugs have been in the engine for a long time, it is better to replace them preventively rather than waiting until they are stuck solid.
How Often to Replace
There is no strict interval like there is for oil or filters. Glow plugs typically last between 80,000 and 150,000 km, depending on quality and driving conditions. If you notice the engine is getting a little harder to start on cold mornings, that is your first signal. Do not wait for a real freeze.
Prevention
There is not much you can do to extend the life of glow plugs since they work under extreme conditions. But you can keep the electrical system in good shape: clean connections at the terminals, a working relay, and a healthy battery all help glow plugs operate under optimal conditions.
If your diesel is getting harder to start in the cold, do not wait until December. Stop by our shop and we will test the plugs in a few minutes and, if needed, swap them out while it is still warm and comfortable to work.