Coasting in gear give me the engine light, C4

Hi all...i know I annoy you all with my recent blast of questions
(only had my vetter for 5 days or so)... but had to throw this at
you...

I notice that about every time I am coasting IN GEAR my "service
engine soon" light comes on.

Example: I live in a neighborhood that sits on a hill. When I leave
home, I throw the car into 3rd gear and just "coast" down, no gas, no
brake.

Doing this in this situation, and any other similar situation for
about 10 seconds or more gives me the engine light...

any thoughts? The only time ive ever jumped the terminals I only got
code 13 (left oxygen sensor), Ive tried a few times since then, but I
dont think I am holding my tounge right, therefore I cant get the
blinking codes...

thanks!
Jazz Mann