What are the signs and symptoms that might make you think your vehicle needs a tune-up? A misfiring engine (when spark plugs ignite at the wrong time) could be caused by worn or fouled spark plugs. Bad spark plugs can also cause low fuel economy, hard starting and sluggish acceleration. Most plugs, though, should last 100,000 miles or more, and engine computers do a remarkable job of compensating for worn plugs, so that might not be the main or only culprit.
Bulbs, though, are probably No. 1 on the replacement list because they're used so much and because there are so many of them. Most cars, for example, have at least three bulbs and usually more on each side for brakes, taillights, backup lights, turn signals and side marker lights. In front, there are headlights, maybe separate high beams, frequently daytime running lights and fog lights, turn signals, "parking" lights and side markers. Chances are that over time at least a couple of those will burn out or stop working because of corrosion or excessive jostling from rough roads.
When to Change Your Car's Battery. There are several signs that indicate the battery in your car is failing and needs to be replaced. Sluggish engine cranking and unusual electrical problems like flickering headlights are telltale signs of trouble, though they may also indicate problems with your car's alternator. You should also replace your battery if it is bulging or has a rotten egg smell.
Of course, there are plenty of wiring problems an engine can experience and they can be very difficult to locate unless the mechanic is experienced. Analysing electrical problems requires using special equipment combined with mechanic expertise. Today's engines are extremely sophisticated combinations of electronics, electrical wiring, and moving parts and seem like they are half-computer and half-mechanical.