Question

On my 1999 Mercury Grand Marquis GS 4.6L, I have the engine codes P0420 and P0455, what is the meaning of the codes?

Answer 1

The P0420 code is for the bank 1 converter.On this car that is the passenger side of the car.

The P0455 is for a large evap system leak. This could be as simple as a loose fuel cap or something more major like a vacuum hose. I would start by checking the gas cap, if it is tight then have a shope do a smoke test. I have also seen the vent control valve stick open and cause this code.

Answer 2

P0420 MERCURY - Catalyst System Efficiency Below Threshold Bank 1
Read more: https://www.autocodes.com/p0420_mercury.html

P0455 MERCURY - Evaporative Emission Control System Leak Detected Gross Leak
Read more: https://www.autocodes.com/p0455_mercury.html