An old oxygen sensor (say, 90,000 miles or more) may still work well enough that it doesn’t trigger the check engine light but could still hurt fuel economy. Engine performance can also be reduced by more serious internal problems, such as valves that don’t seat properly or worn piston rings, or by restrictions in the exhaust system. Because the same symptoms can suggest different problems, and there are often several possible causes and cures, it’s better to consult a professional mechanic than to try to be one if you have neither the experience nor the right equipment to diagnose drivability problems. In short, rather than ask for a tune-up, tell a mechanic what you’re experiencing and ask him or her to find the cause.
ABS stands for antilock braking system, and if the ABS warning light is illuminated in your car's gauge cluster, that means the antilock system has been deactivated because of a malfunction. Your car's regular brakes should work fine, but the antilock feature that prevents wheels from locking up during braking will not work. In addition, if your vehicle has stability control and traction control, those will be disabled as well because those systems rely on the same wheel-speed sensors as the antilock system.
Seeking the Elusive Electrical Problem. Many times electrical problems are the most elusive, because they can occur intermittently. For example, the car engine may not work properly, but only after the engine warms up. Or the ignition works randomly. Perhaps the engine cuts out without notice while cruising down the highway. Then again, your car might have a faulty alternator or a defective wire.
"Heat kills car batteries," according to John Banta, a Consumer Reports project leader and part of the team that tests batteries for the magazine. "Many times in cold climates, your battery fails to start your car on a below-freezing day. The reason this happens is that the heat of the past summers has weakened your battery. When you use it in the cold, the starter requires more electrical current to turn over the cold engine with its thickened oil." Leaving your car parked or stored for long periods of time can also lead to a dead battery that you will need to recharge before heading out for a drive.