One of the more confusing "features" on our cars is the dashboard Oxygen Sensor (OXS) light. This light does not mean your OXS is malfunctioning. That can only be determined by testing it. This light means that 30,000 miles have elapsed since the OXS timer was last reset. It is just a reminder to test the OXS (they usually last about 60,000 miles). So when the light comes on the way to turn it off is to reset the counter.
On my car (well, the way it came from the factory) this counter is in a little black box behind the firewall, right in the middle of the speedometer cable, which of course puts it right in front of the driver. To reset it if your light is on, just find the little plastic cap on the back, and pull it off. Under this is a little white button, which can be pushed in, releasing the ratcheted gears and spring that are closing the switch.
In 1985, along with the new dashboard and cosmetics, the OXS timer was moved to a set of gears clogging up the back of the speedometer itself. To get to this you must remove the top of the instrument cluster cover, I think, and release the ratchet from there. I always wondered why this was done, since it was much harder to to reset.
Read on to find out why...
Last week my lower speedometer cable snapped (darn cars just keep breaking, eh? only 286,000 miles and the speedo breaks!), and I swapped in a spare. I also told the audifans list, just to gripe about the unreliability of Audis (joke - get it?). Well, an ex-Audi dealer technician I know replied saying that this was often caused by gunk, grunge, and goo in the OXS timer box, and that needed to be addressed. So I planned to do that ASAP. The next day, the replacement cable snapped...
So I went out into the field, and removed another spare cable from a parts car, and also fished around in my boxes for a spare OXS timer. I opened up my old timer and what greeted me is shown below. I opened up the donor unit planning to gut it so this couldn't happen again (I can count to 30,000 on my own, thank you!) and someone had already removed half the gears!
Just for general neatness (I don't like having disabled equipment hanging around on my car), I managed to find a one-piece speedometer cable, like the later cars have, but with the screw-on fixing cap at the speedo end (the later ones have a plastic bayonet type attachment for the cable fixing cap). It is part number 811 957 801 E, and when I bought it in 12/99 it listed for about $65.
You can see, the bottom 3/4" or so of the inside of the case and the internal mechanism are a pretty goldish brown color. Guess I know now how deep the water used to get in there!