Many engine-governing valves in cars are electronically operated. Electronically-operated valves allow the engine control unit (ECU) to make real-time adjustments to the fuel/air mixture, spark time and other variables. The gas mileage of the vehicle is directly affected by the ability of the ECU to make those adjustments. Electronic valves are commonly powered by a solenoid, a type of applied electromagnet. With a few basic tools, you can test proper function in solenoids.
Remove the solenoid valve from your vehicle. Attach one alligator clip lead to the negative post of the battery. Attach another alligator clip to the positive post of the battery. If you don't have alligator clips, you can attach two pieces of copper wire to the battery posts with needle-nose pliers.
Place the solenoid valve so that you have access to the wiring terminal and the moving parts are clear. Determine which pins are the positive and ground connections; attach the alligator leads from the battery to the terminal on the solenoid. You may have to refer to a vehicle-specific repair manual. After applying current to the solenoid, it should snap sharply open. On some solenoids --- the starter solenoid, for example --- there are only two or three terminals.
Check the coil resistance with the multimeter. You will have to determine which pins to use to check coil resistance. Any vehicle-specific repair manual should have information about which pins to check. Unfortunately, there is not a standard pin-numbering scheme for automotive electrical terminals. If there is no resistance from the solenoid, it is defective.
Replace the solenoid valve if the coil resistance does not read as specified by the manual; or if the solenoid does not open sharply when an electric load is applied.
Disconnect any power sources. Remove the solenoid from the appliance.
Check the solenoid for resistance. If there is no resistance, the solenoid is defective.
Test the solenoid --- if it uses less than 120 volt current --- as outlined above; attach it to a battery of the appropriate voltage. The operating voltage of a solenoid is often printed on its case.
It is dangerous to test solenoids that operate on standard household current.