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2017 Mercedes Benz c63s spark plug popped out messed up my head
Short story spark plug for cylinder 5 and 6 code p21a0 I had misfire for 5 and 6 checked spark plugs were popped out the spark plug L igniter fell into cylinder head 5 and 6 threads are fine replaced spark plugs for 5 and 6 code went away for 6 now cylinder cylinder five is still there coils are fine fuel injectors are fine I was told the head needs to be done will the head being refurbished fix my problem picture of spark plugs and borascoop picture of head
Something similar happened to an E63 owner not too long ago. He had just done the spark plug service, but one of the new spark plugs was faulty and fell apart. He ended up needing a new engine. Luckily, the service was done at a dealership, so the 1 year parts and workmanship warranty covers the engine replacement. Having part of the spark plug fall into the cylinder will do lots of damage. I doubt it's just the head that got damaged. I would expect the piston is damaged, too, and who knows what else. Where did the piece go? if it's still in there, it will cause more damage somewhere.
What's the story of your car? How many miles? When were the spark plugs replaced? They are due every 50k miles or 5 years on stock tune, whichever comes first. Tuned?
Last edited by superswiss; Jan 23, 2026 at 04:42 PM.
What's the story of your car? How many miles? When were the spark plugs replaced? They are due every 50k miles or 5 years on stock tune, whichever comes first. Tuned?
Interesting questions.
plugs replace 3 years/ 70.000 km says the inspection manual MB.
Okay, the electrode from the spark plugs is broken
end now somewere in engine , cat etc ?
The threads from the plughole ( cylinderhead) are ok you say ?!
On the pictures i see damage on piston from broken electrode.
It looks like you had a borescope camera to look around in the cylinder. If the spark plug ground strap wasn't in the cylinders then it probably exited through the exhaust.
You can see that it bounced around in the cylinders a bit, and so that makes me worried it may have damaged valves or valve seats, tip of the injector, etc.... However, the compression test shows it's at 150 psi. I think that's okay, maybe a little low, but the bigger question is does it hold that pressure or leak? If there is a damaged valve surface or valve seat surface that psi will leak down pretty quickly, and then, yes, you need to remove the head and have a valve job done. Some leakage is normal, because air can get past the rings, but very slowly.
If the spark plug ground strap exited through the exhaust the next thing in the way is the turbo exducer wheel, so when you pop off the downpipe to see if the broken piece is sitting on top of the cat, you should inspect the turbo's exhaust wheel blades.
The whole point of finding that spark plug piece is to make sure it's not still in the engine where it can cause more damage.
The bigger question before you put new plugs in it and give it a try, is why you had 2 spark plugs fail. Were you running low octane fuel? Maybe pre-ignition cause those to break. Are you tuned? Maybe the spark timing map is off.
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