I am new to this forum and new to working on BMW's. I have 1998 540i. It needs oxygen sensors. I bought one front and one back at the BMW dealer as that is all they had in stock. I need to replace all 4. Is there any issue with replacing the two that I have and then replacing the other two later when I can get them? Also, anyone know good sites to use to order these types of parts at a lower cost than the dealership?

Thanks!

Are you certain you need to replace all 4? The rears just check that the cat is working properly. It's the front 2 you need and you can get Bosch ones aftermarket (no need to go to BMW).