1992 E36 325i, check engine light comes on, idle starts surging. I changed the ICV and nothing changed. I changed the O2 sensor and the check engine light and the idle surge went away! Problem solved........except. I have now done this "fix" on 3 separate occasions over the last 6 months. Each time it solves the problem, but each time the problem eventually comes back. No codes other than something isn't reading the O2 sensor. What is it?