One reason a vehicle's horn may not work is a failed horn relay. Testing a vehicle's horn relay is the place to begin when troubleshooting a horn that fails to honk. You can sometimes test a horn relay easily and without tools. A second, more exact method requires special instruments and a power source.
Determine where the relay box is mounted by referring to the owner's manual. Locate the horn relay inside the relay box.
Pull the horn relay from the relay box. In the same relay box, find a relay of equal size. Pull that relay from its seat and plug it in the horn relay's seat. If a comparable relay to the horn relay cannot be found, this method of testing can not be used.
Turn the car's ignition to the "On" position. Attempt to honk the horn. If the car honks, the relay originally seated in the horn relay seat does not function properly.
Find the relay diagram illustrated in the vehicle owner's manual and learn where the relay box is mounted in the car. Find the relay box in the car.
Pull the horn relay from its seat in the relay box.
Adjust the multimeter to the resistance setting.
Identify the ground, voltage, battery and load prongs on the relay.
Fasten one of the multimeter's clamps to the load prong that goes to the horn and one to the battery prong.
Connect the ground terminal from the car's battery to the ground prong on the relay with a double jumper wire. Connect the jumper's other wire to the positive terminal on the car's battery.
Touch the other end of the jumper wire connected to the car battery's positive terminal to the relay's voltage or input prong.
Read the multimeter, check for continuity and listen to the relay for a clicking sound. Continuity and clicking indicate a good relay. No clicking and no continuity are symptoms of a failed relay.