1986 C20 1ton dually conversion Check Engine Light

[kristian_dancer]

Hi Everyone,

I just bought this great old girl I naming Mabel:lol:

She starts great, runs good no drips or leaks, the only thing I have found is that her check engine light comes on over 50.

As soon as I stop and turn her off it goes away.
Oil pressure according to the gauge is around the 30-50psi, temp is good, she has oil and fuel makes no weird or unusual noises any idea?

What does the check engine read on a truck this old?

Oh has no cat dually exhaust from engine back can't see any o2's.

Thanks
for any help you might offer.
Kris