P1421, this might help others

2009 nissan versa, during winter a year ago, was throwing P1421 code, repair manual gave a list of things, did a number of them over the winter period, resetting the CEL after each effort but problem remained. problem mysteriously disappeared during spring/summer/ fall, but same code returned this winter.

started digging into multiple procedures of the repair manual again. removed/cleaned both ends of engine to body grounding strap as well as four wire-bundle-to-engine grounding connections. reset CEL, problem has not returned.

btw, there is a procedure for retrieving and resetting the CEL without using a scan tool and without disconnecting the battery but it involves a stopwatch and patience.