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