How do I reprogram my replacement key

Key cylinder and switch's are a common problem on Honda's and Toyota's. Normally there is a work around if you have your original Black key. We re-key the new cylinder before installing the switch. It sounds like you just bought a replacement from a parts store and had it changed. If you changed the ignition switch and did not have the cylinder re-keyed to your original black key the Powertrain Control Module (PCM) will not have receive the correct code and it goes in to thief mode. Sorry to tell you but the PCM has to be reprogrammed with a Honda PGM tester. If you've lost your original key they will have to contact Honda Customer Support to get your code to start the re-key process. You should spring for at least 2 of the high dollar keys at that time if you plan on keeping the car. Try this to get the answer to it being a key support problem. Turn the ignition switch on but don't try and start the engine. The alarm light will come on for 2 seconds and then go off if the PCM read's the key code correctly and that will tell you there is another problem. If it start's blinking after 2 seconds and you have no other keys like the gray valet key you will have to have it towed to the Honda Dealer for re-coding. You might give Second Generation Locksmith a call and see if if He has the Newest key code equipment for your car. A trip charge from him would possibly be cheaper then a tow truck. I have gotten lucky a few times with older Honda's by turning the key on for 10 seconds off for 30 seconds 3 times and the fourth time leaving the key on for 25 minutes, then disconnect the Neg battery cable for 30 minutes. Then turn the key off and take it out. Make sure any aftermarket stereo or other powered equipment is disconnected. (ie phone charges, GPS, Amp, Aftermarket stereo, anything with a capacitor) Then disconnect the positive cable from the battery and hold the Neg and Pos cable end's together for 60 seconds. This discharge's any residual energy in the PCM, BCM, ACM, ICM and any other system module. Then in order, reconnect the POS cable, The Neg cable, put the key in the ignition and turn in on for 1 second and off for 10 seconds. Three times and the fourth leave it on. Watch the light if it blinks wait 30 minutes if it continue's to blink and the light stay on for 5 seconds after being turned off, sorry but your dead in the water and it will be off to the Honda dealer. Good luck and have a great 4Th of July. TJ, Mingo Auto, ASE Thriple Cert. Master Tech