Where is the engine contol relay on a 2004 hyundai sonata at

You could possibly have a "brake lamp switch" faulty in your vehicle. There is a recall on these for certain model vehicles. Your vehicle may think your brakes are on, which isn't allowing the cruise control to activate.
Also the new Hyundai's are manufactured that if the Engine light is on, your cruise control won't work.
But I would try the Stop Lamp Switch first.

Hope this helps.

Your Sonata is equipped with a "Keyless Entry & Burglar Alarm" system. When the Burglar Alarm is activated, it KILLS the starting system through the used of a relay called the "Burglar Alarm Relay". This mode prevents engine starting with the key.
Solution #1
Disarm the system with the Keyless Remote (key fob) If you don't have this remote (or its battery is dead) replace the battery & try disarming again.
Solution #2
Enter the vehicle & insert the key in the ignition and turn the ignition switch to 'RUN'. Wait 3 to 5 minutes until the alarm stops. You should then be able to start the engine.

Have a qualified/certified Hyundai Service Technician properly diagnose this issue.

Go to www.hmaservice.com and register your vehicle by VIN #.. you'll then have access to wiring diagrams, shop manuals, service bulletins, campaigns, illustrations and procedures, etc.
The horn fuse in in the fuse/relay box under the engine hood along with the horn relay.

A LITTLE MORE SPECIFIC PLEASE,

IS IT THE CHECK ENGINE LIGHT?



FROM MASTER TECH HYUNDAI.

you have a bad relay.. the 'lock' relay.

600CCA

Check for a sticking relay - remove relay to verify.

CHECK THE GROUND SIDE OF THE BATTERY