car - ae86
engine - 16v 4age, JDM loom and dash loom

So about a month ago, the check engine light came on once out of no where while driving up this one bridge
it happened again only ever on this same bridge
since then i have done a complete engine oil change + new filter, checked brake and clutch fluid, and radiator. .all fluids are top level so assuming that is not the case..

then the check engine light happened on a normal straight road while taking off at a red to green light..when the light comes on, the car still drives normal..

so why does the check engine light come on? when it does, its only for about 3seconds then goes away and the car still drives fine, thrashing it and its fine...i can not recall when it happens because it does it at very random times..but so far has done it about 5-6times since it has started..

what things/sensors would trigger the check engine light?