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
)

1-2-1 - fault in signal to or from the AMMM. Could be a faileing AMMM, start by cleaning the connecttions at the AMM.

2-1-2 - O2 sensor signal missing or faulty. The O2 sensor is either on the cat converter or in the pipe just in fron of it. Try cleaning the connection also.

Yes remove fuse #6 for the time period. I have had no luck clearing codes according to procedures in Bentley.

Dan

Try disconnecting the negative battery cable for 15 minutes, to help clear her brain, then see if the code re-sets. The O2 sensor should be there somewhere. Follow the exhaust pipe to the cat converter. Should be between the cat and the manifold, or a telltale plug of some previous nefarious activity.
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Gary Gilliam Sumerduck VA, '94 940 na Regina 160k '86 240 190k

Sumerduckman wrote:

> Try disconnecting the negative battery cable for 15 minutes, to help clear her
> brain, then see if the code re-sets.

Oh, I so don't want to have to do that, and for particularly pedestrian reasons: I have what once was quite a nice Blaupunkt in there, but one whose display has gone black. If I disconnect the battery, It'll be such a pain to reset my presets that I'll probably just end up buying a new stereo. That's a hefty investment just to reset my check engine light, so that'll be a last resort.

Besides, I did the standard reset procedure, but still came up with the same codes, so I figure the codes are valid, no?

I understand pulling the number 6 fuse cuts power to the ECU. Would that reset the codes?

-EdM.

"I understand pulling the number 6 fuse cuts power to the ECU. Would that reset the codes?"

Yes, it will.

Not all codes set the Check Engine light, But those two do.

The RPM sender harness is in the vicinity of the O2 sensor connections, but I hardly think you could mess things up there. I have no clue about the AMM code. Try reseating the AMM connector and see if the code comes back.
--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.