Many thanks in advance to all who contribute ideas here, I'm scratching my head.

I bought a 1996 850 last year that had not been taken good care of, and was gradually bringing the maintenance up to date, but over the holidays took it on an out-of-town trip and busted the timing belt while traveling. Because of the "interference" design, this wrecked the engine. I've now figured out its called an interference engine because when the timing belt breaks, the valves "interfere" with the piston heads. Not good. Expensive lesson learned, don't skip the 75K mile timing belt replacement schedule.

I purchased a relatively low-mileage 2.4 non-turbo engine out of a 1998 S70, from a very reputable outfit here in Atlanta that I've used for years, and the engine is now installed and seems to run well with the exception of a "check engine" light. There are some minor differences in the fuel rail and the intake manifold of the 2 engines. The 98 fuel rail does not have a return hose to the fuel tank, where the 96 has a pressure feed and a return. I left the single-hose rail on the replacement engine and have plugged off the return hose. The other difference is where the EGR valve assembly mounts to the intake manifold.

After installing the engine, I realized (dohhh!) that the location where the EGR valve was mounted on the original manifold is covered by a plate on the new manifold, so I transferred the original manifold to the new engine. The primary reason for doing this originally was a "check engine" dash light and that two codes were coming up on the OBD2 reader (1) P0326 Knock Sensor 2, Bank 1 or Sensor 1 and (2) P0331 Knock Sensor 2, Bank 2.

I assumed that the codes came up initially because the EGR valve was missing, but after swapping the manifolds and reinstalling the EGR valve, the codes were still there. You can reset them but they come back in almost immediately after starting the car.

I've checked everything I can think of. Any ideas or similar stories would be greatly appreciated.