P0420 Toyota - avalon 2002

Question

IF THERE ARE CODES SUCH AS PO440, PO441 and PO446 do they need to be resolved first than see before trying the 420

Answer 1

P0440 Evaporative emission control system malfunction

P0441 Evaporative emission control system incorrect purge flow

P0446 Evaporative emission control system vent control system malfunction

P0420 Catalyst below threshold Bank 1

Yes these code should be resolved first, before trying the catalytic code, When you have 3 evap codes either case has happened, the fuel cap is not sealing properly or, you have a disconnected purge hose that mayhave  came out during a airfiter replacement or inspection, without the purge you would code a incorrect purge, and a incorrect vent. and this does affect the intake airflow which can upset the fuel trim and set a P0420.

Answer 2

P0420 can only be set if there are no oxygen sensor codes as the upstream and downstream o2 sensors are compared. When both sensors are reading about the same this means the cat is not storing and releasing oxygen to do chemistry. Evaporative emissions displacing oxygen could affect this potentially. So fix the less expensive items that affect oxygen availability first. Then clean the cat with water ingestion only while engine is running or a cat cleaner in the gas tank... Usually you have to replace the cat.