I recently failed emmissions with two trouble codes: p0420 and p1456. I took it to precision auto care and spent 300$ and have to return for more tests including smoke test and diagnostic test to find exactly the problem...if it is sensor or needs replace the entire catalyst conv. I would like any info and advise that you have PLEASE... 99 Honda accord... both my maintenance and engine lights are on...

 

you can reset your maintenance light yourself
if your referring to the light next to where it says unleaded fuel only, this is just hooked up to your odometer the first code p0420 is you catalytic converter
A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
the p1456 code is your evap system (Evaporative emission control system)