i have a 90 engine mounted on a 70 mustang.
idling is rough, power is low and i cant run the koer test with the snap-on.
what checks should i do in order to replace the right part and not all electrical parts?
the hego sensors works fine ( i checked them with avometer to see they switch fron lean to rich).
map gives me 2.5 v with koeo.

:)

Try jumping the connections to get the KOER code without the tool. This way you can rule out the tool being at fault.