I agree with Sweenewl to check for the bad Glow plug (GP) instead of replacing them all. A resistance check will detect a bad GP if it is open but not always if the GP is marginal and a small difference in resistance may not be detectable. A current test is better because it checks the GP hot compared to the resistance check which tests the GP cold. You need a 20 Amp DC meter. Any GP that reads a lower current than the others should be replaced.

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