So, I just replaced the crank position sensor and fuel pressure regulator on my '90 240DL Wagon. When the job was done and I started the car, my check engine light came on and stayed on. I checked things out at the OBD port (something I've never done before, but learned to do from this board), and came up with the codes 1-2-1 and 2-1-2. Tried resetting it, and still came up with the codes. The FAQ says that means faulty wiring at or in the oxygen sensor and air mass meter.

Now here's the first odd part: the codes have nothing to do with the stuff I replaced (I figured it would have something to do with the fuel pressure not having come back when I started the car again), yet I didn't get a check engine light before. Not knowing much about the OBD system, I wonder if someone could fill me in: do these codes always result in the check engine light's coming on? Is there some other reason the check engine light would be on without a code (I don't know if these codes were present before I did the work)?

And here's the other odd part: I can't for the life of my find my oxygen sensor. Shoudn't it be poking out of the exhaust manifold somewhere? I can't even find a wire going to it, and there doesn't seem to be room under the heat shield for it to be there. It's not on the headpipe, either, but I seem to recall having that replaced (gotta check my records). What are the chances that it is just not there, due to some faulty repair work in the past (I'm the third owner). If indeed it is not there, that would certainly explain the 2-1-2 code, but shouldn't I have always a persistant check engine light? And where exactly should the connector be on the firewall? Perhaps if I can start there, I can trace the wire.

-EdM.
'90 240DL Wagon "Lola"
'72 1800ES "Galadriel" (No OBD--real human diagnostics
)