Start at the basics and forget that the plugs were recently changed...that doesn't mean one isn't bad which is what it sounds like is wrong. Test the GPs with an ohm meter and see what they read in resistance. You can do it from the relay plug and there are many posts elsewhere on this forum disucssing how...then you will know if it is GP or the relay that is bad.

How does it run after a cold start? Is there roughness for a while, like one cylinder is misfiring? That would also point to a bad GP. Also, not reaming out the holes can lead to premature failure I have heard. Do you know if they cleaned out the holes?

Finally, if the work was done at a MB dealer 5 months ago I'd take it right back to them. Parts are guaranteed for 12 months and their labor ought to be redone as goodwill if you now have problems with either the GPs or relay.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz