Signs of a poorly tuned engine: lack of power, poor idle, hard to start when cold or hot, poor gas mileage, or engine 'just doesn't feel right".
Damage? Depends on what they did, but usually it's more of an inconvience than anything else. Of course if left unchecked for a long period of time... that's another story.AnswerAll depends on what they did wrong. not treating the wires, wrong heat range or wrong speced plug that non-platinum, single platinum or double platinum yeilds premature failure, poorer fuel economy, miss, no start, strain on ignition coil or module). Wrong threaded plugs yield cylinder head damage. If plugs were cross threaded or overtightened or broke installing can wreak all kinds of havoc. mis installation of of fuel filter could result in fire, not closing or breaking the air filter housing can give a check engine light. Mis installing certain PCV valve can give abnormal noises. The list can probably go on for quite a bit.
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