My 2001 300m just turned over 100k last week. A few days later at about 100,220 miles the check engine light came on, first time since I bought the car with 17k on it.

Is the check engine light programmed to come on at about 100k to remind of the timing belt/ water pump service?

get the code read see what it says,

I knew that answer was coming! I have an appointment to take the car for service tomorrow, wanted to know beforehand what I was in for.

DO the on-off-on-off-on-off-on trick. p0446 is the gas cap.

In Michigan you can pull up to any AutoZone or Pepboys? and they will politely come out and put a mini computer reader thingy on you car and most times tell you what's wrong...for free.
The idea is that you go back inside and buy the part

I agree they just read the codes.they wont spend 3K for a scanner to actually see what is going on.Sometimes you get lucky and and sometimes you dont.Ive seen people spend all kinds of $$$ because they dont want to pay for a diag.Then after blowing $$$$ they pay to have it fixed right.Rocky

It threw two codes using the on/off, on/off, on/off sequence. One was P0340 which the shop manual listed as "no cam signal", so I supposed that meant cam position sensor was gone.

The other code I can't remember at the moment, "P16??" did not appear in the shop manual (at least it was not listed with the other possible codes). I'll get back to my notes tomorrow.

Replace the cam sensor for the P0340.

The code I couldn't find in the shop manual was P1684.

Correct, new cam sensor cleared the P0340.

okay, found P1684 means battery disconnected within the last 50 starts.