'90 F150. - Red Engine Light -vs- Yellow Engine Light

1990 F150 , 5.0 v8, 4x4 , longbox.
Bought it recently for $500. Truck has 170k miles, "Rebuilt Engine" has 65k. Rusty, but runs strong and starts immediately and doesn't
burn oil. No regrets.
I don't have the manual, and the truck has some "post operative" stupidities left over from the engine transplant. The extra set of battery cables under the hood was alarming. The oil and temp guages are dead. Missing hoses (different thread) and exhaust fun (different thread) indicate that some barnyard mechanic did the engine replacement.
Anyways, I can point at reasons for at least one 'check engine' light being on, but am at a loss as to why there are two and how they differ. For the record, the red one is always on, and the yellow one comes on very rarely.
So, whats the yellow one for? RED = HELP YELLOW = I REALLY MEAN IT
Thanks Mike D
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Yellow or orange are really for emissions I think. Red for oil pressure or temp.
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OK . The oil and temp guages are dead and I'll bet the sensors are dead or disconnected, so the red lamp lit makes sense. The yellow only lights up rarely when the engine revs real low under strain (right before it downshifts to climb a hill), which also makes sense since the air pump to the catalytic is disconnected. I suppose I should look at the bright side.. The Oxy Sensor is probably working ..
Thanks for the info.
Mike D
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