oxy sensor light on.....???

Well I had the car out yesterday afternoon its been about 2 weeks since I previously had driven it. Its an 81 911SC I noticed that the oxygen sensor light on the dash was barely lit and would not go out, the car runs like it always does ...great. What could the problem be?? The light is very dimly lit. Thanx in advance. David

it lights up at 15K intervals. pop out the speedo a bit and about 10" down in the hole is the counter box. square, black, plastic. push the white button with a long screwdriver to shut off the light. it's a service interval reminder.