PO420 is the code for a bad cat typically, but there may be transmission codes or others.
Most of the time a plugged cat acts like a governor and limits the engine RPM to a fast idle. Combustion must have a release which is the exhaust. If the engine can not gain momentum because it can not exhaust, the transmission will have no reason to shift.
There are two ways that you may be able to check this besides what I already mentioned. Remove the front oxygen sensor so there is less restriction or better yet disconnect and support the front pipe. Therefore no restriction. If it runs good at that point you verified the cat. Or plugged exhaust of somesort.
Friday, April 9th, 2010 AT 7:39 AM