The horn is a safety feature on your Toyota Sienna, and you may not think about it until you need it and it's not there. There are a number of possible causes for a malfunctioning horn, and you'll need to perform a number of different tests to locate the exact cause and determine how to repair it. The Sienna's horn is usually mounted near the radiator. This process may vary depending on the van's exact year.
Disconnect the horn's electrical connector.
Connect a battery directly to the horn's electrical terminal using a jumper wire to supply voltage directly to the horn. Replace the horn if it does not sound.
Connect a voltmeter to the horn terminal if the horn does sound from the earlier test and and check for voltage while another person presses the horn button. Look for a bad ground wire at the horn if there is voltage.
Check the relay for the horn if you get no voltage -- the relay is located in the fuse box -- as per the instructions in Section 2.
Check for voltage to the relay's power and control circuits using the voltmeter if the relay is good. Check the wiring between the fuse panel and the relay if you don't get voltage at either circuit.
Press the horn button and test the circuit from the button to the relay for continuity to the ground. Look for an open circuit if there is no continuity; replace the horn button if there is no open circuit.
Connect a fuse jumper wire to one of the relay's two control circuit terminals (the terminals should be labeled) and connect the other control terminal to the battery's positive terminal with a another wire.
Listen for a clicking sound from the relay with the above the connections. Switch the positions of the jumper wires if you don't hear a click, as you may have the wrong polarity. Replace the relay if you still don't get a click.
Check for continuity at the power terminals while the wires are connected to the control terminals. Replace the relay if there is no continuity.
