Check Engine light intermittent 88 4runner 22RE

I have an '88 4Runner (200K miles) with intermittent check engine light problems. I replaced the O2 sensor and it got better. It seems to come on more often lately and is now on most of the time at highway speeds. Mileage seems low when it is on, probably computer default to basis settings? What can cause the check engine light to go on on this vintage 4 Runner? Are there vaccuum hoses on the emissions control system I should check? Is the diagnostic tool specific enough to isolate the problem? Who makes the most suitable and affordable one? What simple tests will work without diagnostic tool? Thanks