GMC Repair: Knock sensors on a Yukon XL, 2003 yukon xl, radiator fluid

Question
I own a 2003, Yukon XL.  In January, when I started my car, there was this high pitched constant chime.  I looked on my dash and I noticed I had no oil pressure.  I immediately turned the car off and had it towed to a car shop.  They told me 1st replace the pick up tube and if that doesn't work they'd replace the oil pump.  If that didn't work they might have to replace the engine.  I had pick up tube placed, didn't work had it towed to the dealer.  Was told radiator fluid was getting in engine, although my vehicle was driving perfectly up to that point, perhaps this caused oil pump to go out. Had engine replaced with a used engine costing $3,500.00  Within 24 hours after picking up vehicle, engine light came on. Took it to dealer who now tells me my knock sensors need to be replaced and it would cost me another $400.00  Is this something that should have been covered with engine replacement?

Answer
Some shops do replace them, most sensors. I would have. If radiator fluid was getting in your oil, it could have been a cracked block. Then again it could have been a head gasket or only a cracked cylinder head. 400.00 sounds a little steep, but it is necessary.