1999 Honda civic si, idle issues

Hi, my name is Devin. I am an auto repair expert with the Car Genius team here at RepairPal. I'm sorry to hear you're having trouble with your vehicle. The idle issue you're experiencing with your Civic, which runs fine without the lights and heater but becomes erratic when they're on, may indicate a problem with the electrical system. Turning on the lights and heater increases the load on the electrical system, which could cause the idle to rise if the IAC isn't compensating properly. Since you've recently replaced the alternator and battery, it's worth checking that the electrical connections are clean and secure. If you need more advice, have any other questions about your vehicle, or would like to get connected to a RepairPal Certified shop near you, we have you covered. Give us a call at 877-913-1554, and we will happily help you out!