How to Test if a Glow Plug is Good or Bad?
4 Answers
If there is resistance, it indicates that the glow plug is intact. If the resistances are basically consistent (error rate less than 2%), then the glow plug is 100% intact. Generally, the resistance ranges from 1 ohm to 5 ohms, varying with different vehicle models, and the lower the value, the better. If there is no resistance (the multimeter shows 0 or very close to 0), it indicates a short circuit, meaning the glow plug is damaged. If the resistance is infinite, it proves an open circuit, meaning the glow plug is damaged. Precautions for using automotive glow plugs: 1. Function of glow plugs: Glow plugs are used to preheat the intake air during cold starts of diesel engines. Using glow plugs in winter can shorten the start-up time, reduce battery current consumption, and minimize diesel engine wear. 2. Cold engine power-on: When the engine is cold, power the glow plugs for 2 minutes and touch the electrode of each glow plug by hand. If the heating element of the glow plug is broken, the electrode will be cold. This method is simple and easy to perform but slightly less effective. 3. Individual testing: For individual testing of glow plugs, remove each glow plug, ground the outside of the glow plug, and connect the live wire to the glow plug terminal F (use 12V power for vehicles with a 24V battery) for a short-cycle test. If the glow plug becomes hot, it indicates that it is intact.
Glow plugs are mainly used in diesel vehicles to help the engine start more easily in cold weather. A reliable way to test their condition: first, disconnect the power supply for safety, then use a multimeter set to the resistance mode to check the resistance value of each glow plug one by one. Normally, it should be between 0.1 and 6 ohms; if the reading shows infinity or zero, it's definitely faulty and needs replacement. Some auto repair shops have specialized glow plug testers—plugging them in to see if they heat up also works. Remember to handle with care to avoid short circuits, as unresolved starting issues can leave you stranded in winter. As an experienced driver, I've encountered this situation a few times; after testing, I found it was just loose wiring, and tightening it myself saved me repair costs.
Testing glow plugs isn't actually difficult to do yourself - I'm just an ordinary car owner who enjoys DIY. Here's a simple method: After turning off the engine, open the hood and check the glow plugs for any cracks or burn marks. Then turn the ignition key to the ON position without starting the engine, and listen for a slight buzzing sound near the glow plug area - no sound might indicate a faulty plug. You can also use an inexpensive multimeter to measure resistance: touch the probes to both ends of the glow plug, and if the reading is too low or too high (e.g., over 15 ohms), it's likely defective. Don't forget to check if the battery voltage is stable, as voltage fluctuations can affect the results. I recently had slow starts with my car, tried this method myself, replaced the faulty plugs, and it started smoothly immediately. The tools cost just a few dozen bucks online.
Glow plug testing can be easily done with tech tools. Set the multimeter to resistance mode, and a reading around 1 ohm indicates it's good. Dedicated testers are faster but cost money. Checking if the engine starts smoothly after preheating is also reliable – difficulty starting often means the glow plug is failing. Regular testing prevents cold-start issues in winter.
