98 Avalon engine check light intermittent

The engine check light on my 98 Avalon with 170,000 miles on it has started to go on and off every few hours or sometimes days apart.
Engine works fine.
Had the code read 3 times and every time is shows a different issue. Could it be a potential bad contact or ????
Any suggestion shall be welcome. Thanks

Please post the codes which have been stored. Each one is potentially relevant.

Thanks for your input.
While I do not recall the numbers given to me by the shop, the 1st and 3rd time was the oxigen sensor and the 2nd time was ECT.
I did have to change the sensor a couple of times, but when it went bad I remember the check light will stay on until resetted.
Mechanic asked me if I noticed any problem with the auto transmission, to which I said no problems at all either in normal or ECT mode.
It has been well over 2 months now with the check light intermittently lighting up and no changes in engine or transmission behavior has been detected.
Alberto