The complexity of modern vehicles makes some projects a no-go for newbies, veteran Portsmouth, R.I., mechanic Paul Roderick says. Don't try to replace fuel or water pumps (because of their inaccessibility) or the shoes in drum brakes (because of their mechanical intricacy) unless you really know what you're doing, and "forget about electrical problems, unless the problem is a blown fuse." But, Roderick says, there are still plenty of auto DIY projects, like these, for weekend warriors.

Changing Disc Brake Pads

Changing worn-out disc brake pads is a straightforward job. The best way to tell if they need replacing is a visual inspection. Some pads have a groove that serves as a wear indicator; if not, look through the caliper to see if the pad looks thin. Otherwise, some cars have a warning light to let you know if the pads are nearly worn down to the rivets, while others have brakes with a strip of metal that starts to squeal against the rotor when the pads are worn way down.

When it's time to change the pads, make sure you have jack stands, basic hand tools, and a C-clamp (to push the caliper piston into its cylinder) on hand. If badly worn pads have severely scored the rotor, this would also be the logical time to replace that; it slips off the hubs once the caliper and its bracket are removed.

Replacing the Alternator

In most cars, a warning light will tell you when the alternator is dying. If not, you'll know soon enough when your car stalls (from running off a depleted battery) or won't turn over when you try to start it. Novice mechanics can replace dead or dying alternators as long as the alternators are accessible from the top of the engine. (If they're down around the bottom of the block, they can be impossible to see and you have to work by feel to get them out and replace them; that takes experience.)

To do the job, you'll need wrenches, including a socket wrench. Some cars might also require a special tool to take the tension off the belt. And be sure to do things in the proper order. Unbolting the alternator before detensioning the belt could cause both of them to bind up so that you won't be able to budge either of them.

Replacing the Oxygen Sensor

A common problem that trips the Check Engine light on your dashboard is a faulty oxygen sensor. This is one of the many times you'll need an OBD II scan tool, at least if your car is a 1996 or newer model year. If the scan tool points to an O2 sensor, be aware that vehicles can have as many as four O2 sensors in the exhaust system—one or two before the catalytic convertor and one or two after. They look like spark plugs, and your diagnostic tool might be able to tell you which one needs replacing. (If not, you'll need someone with a higher-tech scanner to identify it.)

The O2 sensor can be removed with an open-end wrench, but you should buy a socket specifically designed for the sensors from your parts dealer because it will have a groove for that outgoing wire. Spray some penetrating oil on the base of the sensor before attempting to turn it with a wrench; it'll make the job easier.

Replacing the Idle-Air-Control Valve or Sensor

If your car is idling poorly and stalls when you put it into gear, you could have a faulty idle-air-control valve or a faulty sensor. (You'll have a Check Engine light glowing, too.) If your OBD II scan tool identifies the sensor as the problem, simply follow the hose leading out of the air cleaner to the throttle body, which is on top of the engine; it controls the flow of air into the fuel-injection system. Replacing the sensor requires only removing a pair of screws. If the diagnosis indicates that the valve itself is faulty, you're still in business: the valve is also held in place with just a pair of screws.

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