A number of the power loss-related problems we just listed will cause your malfunction indicator lamp, aka the check engine light, to switch on. Heed it. Many of the issues it detects relate to power loss -- including problems with the catalytic converter, the mass airflow sensor, the O2 sensor or the spark plug wires. More important, small problems often hint at larger ones [sources: Consumer Reports; Reed].
In addition to the glitches already cited, your car's MIL could point to a bad positive crankcase ventilation valve. The PCV valve is part of your emissions control system. When it sticks open, too much air pours into your engine; when contaminants gum it up, your engine suffocates. Fortunately, the valve is fairly easy to clean or replace [source: Sclar].
If you're working on a 1996 model vehicle or later, you can buy a cheap trouble code reader that will tell you what the MIL means, possibly saving you a trip to the mechanic -- or at least arming you with more data before you haul in your vehicle [sources: Consumer Reports; Reed].
Lastly, if none of these five signs are occurring for you but you're certain you're losing power, check your catalytic converter. Although most converters are designed to last a vehicle's lifetime, they can eventually fail or plug up, suffocating the engine by preventing fouled air from being expelled through the exhaust [sources: Salem; Sclar].