Q&A for How to Test Glow Plugs

To change glow plugs, start by disconnecting the wire from the end of the glow plug. Then, using a socket wrench, carefully back out the glow plug as you would with a spark plug. However, it's important to note that sometimes, when a glow plug fails, the tip may swell and expand. This can cause difficulties when removing the glow plug, as the hole it fits into may become smaller due to the expansion. In such cases, you may need to break off the tip of the glow plugs to facilitate the removal. This can leave the broken tip in the combustion chamber, posing a challenge. There are two methods to address this issue. One approach involves cranking and rotating the engine until the broken tip is expelled. However, this method carries the risk of damaging valves and cylinder walls. Alternatively, you may need to remove the cylinder head to extract the broken piece from the combustion chamber. When changing glow plugs, it's ideal that none of them have expanded tips that could break off, simplifying the replacement process.

Check your relay. In older vehicles, you can hotwire a line from the battery to the low-voltage terminal and you should hear the relay click on and off with the application of power.

Glow plugs are in direct contact with combustion gases and temperatures at all times regardless of whether they're being used or not. This means that they are exposed to ambient gases that can exceed thousands of degrees depending the scenario. If one is bad, the others are not far behind, and replacing all of them is suggested for this reason.

As a rule of thumb, if one is bad the others are not far behind. It's better to have a whole new set than just one new glow plug.

Yes. Always plug your diesel in on cold nights for easier starting.

You need a wrench with the appropriate socket to match your glow plug. Hardware stores sell sockets specifically designed to remove glow/spark plugs. Check your engine or your owner's manual for your size.

Under 1 ohm, preferably. Anything higher or no reading usually indicates a bad plug.

Not all the time. Hard starting on colder days is a sign they’re bad. Change them as a preventative measure.