I have seen similar issues when the lash adjustment on the injector rockers is too tight. The injector plunger ends up going passed its maximum position, throwing an 'implausible signal' code, and even shutting down the injector. Try taking the valve cover off and adjusting the lash for the injectors and see if that helps. There are plenty of How- to threads about this with pictures, but basically you turn the engine over to the point that each injector is pushed as far down as it goes, loosen the jam nut, spin the adjuster all the way down until it bottoms out, and then back it off 180 degrees, and torque the jam nut. On the car I worked on, the particular injector that was throwing a code only had about 120 degrees before the adjuster bottomed out.