I had the same thing happen to a Bosch plug. Replaced with a non Bosch plug and it failed shortly. Removed the plug and it looked bradded on the end. Checked the relay and it worked as should to the other plugs but did not shut power off at the failed plug. It was glowing full time, hence the quick failure. If your plug fails again soon check your relay. Good luck.

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1982 240D Euro Stick 4