I recently had the PO 420 code show up on my Grand Marquis. I've replaced the downstream O2 Sensor on bank one. It didn't fix the problem and the PO420 code showed back up. I've replaced the Cats on the left side and it still didn't fix the problem. PO420 showed back up again. I've checked the pins & connections on the CPM and all seems to be fine and I can't find any vacuum leaks anywhere. Any ideas as to what else the problem could be?