Overview
Glow plugs are essential for cold-starting a diesel engine. If your diesel is hard to start in the morning, cranking for a long time, or showing a glow plug warning light, one or more plugs may have failed. This guide shows you how to test each glow plug with a multimeter — the quickest and easiest method.
What You'll Need
Digital Multimeter
Set to resistance (ohms) mode. Any basic digital multimeter will work for this test.
View on Amazon As an Amazon Associate I earn from qualifying purchases.Method 1 — Resistance Test (In-Car, Quickest)
This test checks each glow plug's resistance without removing them from the engine. You need to disconnect the electrical connector from each plug.
Let the engine cool down completely
Never work on glow plugs on a hot engine. Allow at least 30 minutes after the last run, or ideally test first thing in the morning when the engine is cold.
Locate the glow plugs
On most diesel engines, the glow plugs are positioned along the top of the engine — one per cylinder. They'll have individual electrical connectors (small bullet-style connectors) or a bus-bar linking them all.
Disconnect the glow plug connector
Disconnect the electrical connector from the first glow plug. On engines with a bus-bar, carefully remove the bar from all plugs to access each one individually.
Test with the multimeter on resistance (Ω) mode
Place one multimeter probe on the terminal of the glow plug and the other probe on a good earth point on the engine block. A working glow plug will show a very low resistance — typically 0.5–2 ohms. An open circuit (OL or infinite resistance) means the plug has failed internally.
Test all plugs and note the readings
Test every glow plug in turn and write down the resistance of each. All readings should be similar. A plug showing significantly higher resistance than the others, or OL (open circuit), is faulty and should be replaced.
Reconnect or replace as needed
If one or more plugs fail the test, replace them. It's good practice to replace all glow plugs at the same time if the engine has high mileage, as the rest are likely near the end of their life too.
Want this guide offline — and 15 more like it?
Our PDF guide collection covers servicing, fault codes, buying a used car and more. Written by a qualified mechanic. Download once, use forever — no ads, no internet needed.
Fast Diagnosis, Easy to DIY
Testing glow plugs takes around 15 minutes and can save you an expensive diagnostic fee at a garage. Once you know which plug (or plugs) have failed, you can order the right replacements and fit them yourself — or at least walk into the garage knowing exactly what's needed.
