Question

What could cause a p3400 p521 and a p174 code in a 2008 g8 gt 6.0l v8 ls3

Answer 1

P0174 Fuel trim lean Bank 2

P0521 Engine oil pressure (EOP) sensor circuit low voltage

P3400 Cylinder deactivation system performance

When these condition occurs there is no cylinder deactivation from a V8 to V4 mode this is do to P0521, If the engine oil pressure is high the voltage of the EOP is high if the oil pressure is low the voltage of the EOP is low. Check the oil pressure   the oil pressure at 1000 RPM should be 24psig at 2000 RPM should be 35 psig, You will need a oil pressure tester, not the engine gauge.