On an 88 the light is controlled by a timer module that-s under the dash, approximately above the driver-s right knee. It is independent of the ECU and doesn-t read any sensor input -- it just counts down.
Step one is to replace the O2 sensor. Then, the factory wants you to replace the timer. However, there are write-ups on the Internet (there-s probably more than one on these forums) explaining how to reset the timer with a paper clip. You can simply unplug the timer, but then you need to keep track of the mileage on the O2 sensor manually. It should be replaced every 75,000 miles.
