- No spark plugs last 400K.
- It may "look" OK but the performance (good spark) is degrading slowly..
- For Iridium spark plugs, most pros recommend changing it every 70K-80K etc.

- If you look CAREFULLY, the wear is on the PRONG side and not so much on the electrode side.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT