Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSTOYOTA2004CELICA GT, STANDARDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 130 (BODY CONTROL SYSTEMS)SYSTEM TESTSTEST NO. 50: ONLY ONE HORN OPERATES
2004 Toyota Celica GT, Standard
Test No. 50: Only One Horn Operates
2004 Toyota Celica GT, StandardSECTION Test No. 50: Only One Horn Operates
WARNING: This page is about a different car, the 2002 Toyota Avalon. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:
Three different horns are used. One theft deterrent horn is located on passenger side strut tower in engine compartment. Two standard horns are mounted at each side of vehicle, just behind front bumper. Theft deterrent horn is entirely controlled by theft deterrent ECU. Standard horns are controlled either by pushing horn switch or by body control ECU when theft deterrent system has been activated. For testing procedures covering theft deterrent horn and theft deterrent system, see ANTI-THEFT SYSTEMS - AVALON
article in ACCESSORIES & EQUIPMENT.
- Turn ignition off. Remove body control ECU from driver side junction block. See BODY CONTROL ELECTRONIC CONTROL UNIT under REMOVAL & INSTALLATION. Using a DVOM, check voltage reading between 12-pin driver side junction block-to-body control ECU connector terminal No. 10 and good body ground point. See Figure. See HORN in SYSTEM WIRING DIAGRAMS article. With horn button depressed, voltage should be below 1 volt; with horn button released, voltage should be 10-14 volts. Check Black wire between battery and horn relay. See POWER DISTRIBUTION in SYSTEM WIRING DIAGRAMS article. Voltage should be 10-14 volts. If wires, harnesses and connectors are okay, go to next step. If wires, harnesses or connectors are not okay, repair as required. After repairs, perform confirmation test. See CONFIRMATION TEST .
- Check wiring harnesses and electrical connectors between battery and engine compartment junction block, between engine compartment junction block and driver side junction block, between engine compartment junction block and horns, between driver side junction block and body control ECU, and all power and ground connections. See HORN and POWER DISTRIBUTION in SYSTEM WIRING DIAGRAMS article. If wiring harnesses or electrical connectors are defective, repair as necessary. After repairs, perform confirmation test. See CONFIRMATION TEST .
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.