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3 Steps to Replacing Diesel Glow Plugs

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Bosch Glow Plugs CoolDrive iShop

1. Identify Glow Plug Defect

The service life of glow plugs is mainly dependent on the frequency of the starting process and the warm-up phase, and less on their running performance. Short-haul vehicles are more susceptible to wear and should be inspected at an early stage. 

Engine is noisy, uneven and lacks power?

Defective glow plugs can be identified by the following symptoms:

  • Increased smoke formation, especially after cold start
  • Loud combustion noise before reaching operating temperature
  • Uneven running despite a warm engine
  • Decrease in power or increased fuel consumption

We recommend checking the glow plugs after 80,000 - 100,000 km at the latest and changing them if necessary. This will prevent damage to the engine.

2. Check Glow Plug Function

To test the voltage, we recommend using a Digital Multimeter.

Before measuring

  • Clean the contacts on the glow plug and motor housing
  • Determine the inherent resistance (offset) of the multimeter

For functional testing

  • Place the electrodes of the measuring instrument on the connector of the glow plug and the motor housing
  • Read glow plug resistance
  • Resistance from 0.2 Ω to 5.0 Ω means Glow Plugs are functioning
  • Deviating values/lack of voltage means Glow Plugs require replacing

Important: Please do not check the glow plugs using the vehicle battery or a 12V glow plug tester. This leads to an immediate failure in modern preheating systems.

3. Glow Plug Replacement Necessary

Glow plugs need to be checked regularly to avoid damage to the engine. Otherwise the following could occur:


  • At high mileages, the thread of the glow plug fuses to the cylinder head


  • As the engine gets older, glow plugs corrode and literally “rust”

In both cases, there is a risk that the glow plugs will break off in spite of careful attempts at loosening. The lower part then remains in the cylinder head and can only be removed by disassembling it. This will cost a lot of time and money.

Tip: You can easily prevent glow plugs from fusing by loosening them slightly during the test and then tightening them again.

Did you know that glow plugs usually reach their wear limit in quick succession? For the customer, replacing the complete set is more cost-effective and convenient than replacing individually defective glow plugs.

Use glow plugs from Bosch to ensure high quality and long life, find them on iShop HERE.

This material has been prepared with the intent to provide a resource of reliable information; however no warranty (express or implied) is made as to its accuracy or completeness. Neither is any liability assumed by CoolDrive Auto Parts for damage or loss resulting from reliance on the information or process disclosed.