Question: how do you know if you need to replace spark plugs?
Answer: The car is hard to start. The battery is often blamed as the culprit for a vehicle not starting. ... 
The engine misfires.
The car gets poor fuel economy. Worn spark plugs create extra work for your engine. ... 
Rough engine idle. ... 
Your car struggles to accelerate.
The engine is really loud. ... 
Your 'check engine' light is on.