it is 1998 z34. by turning the key, volt, airbag and some other lights would be on. but the starter wouldn't work. the mechanics used a wire to connect something in the hood and starter could be on everytime but it was dead after several seconds. they replaced my battery. however, it didn't start. they have been testing it for a whole day and said it sometimes could start but sometimes couldn't. they said the computer didn't give electricity and fuel and they wanted to replace the ECM. I am wondering if it is the key, since the security light will flash while other lights is on still. What should i do now? I have had a battery replaced which shouldn't be.
I don't know car at all. Would you please give me a simple instruction that what I need to do? If I need to obtain a new key, what the procedure? I only have one key. Thank you

 

ECM and PCM could both cause the problem. The security could be doing it as well. See if you can get some of those items at a boneyard on cheap.

 

thank you very much. hope i don't have to replace them all