1997 Other Plymouth Models Service Light Comes On

That is pretty much what it means. Depending on your readings, the cat might not be doing enough to clean it up, that is why the code and the failed emission test

Is the 420 code still gone? Or is it back? Yes it does mean the converter and a posible sensor 02 downstream of the cat. It means that 02 sensor is switching between rich and lean too much. You can cut the HC's by changing the oil, and plugs. Don't know if you can cut it enough tho. You also want to make sure that misfire problem is fixed, before the new cat goes in, it could destroy it!

Misfires can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.

A Random Misfire code usually indicates a vacuum leak or bad gas.

If a misfire in a specific cylinder should lead you to check the spark plug, fuel injector and compression.

Hydrocarbon failures mean unburned gasoline is passing through the engine and entering the exhaust. The three most common causes include ignition misfire, lean misfire and low compression (typically a burned exhaust valve). Ignition misfire can be caused by worn or fouled spark plugs, bad plug wires or a weak coil. Lean misfire results where there is too much air and not enough fuel, so check for vacuum leaks, dirty injectors or a fuel delivery problem. In addition to these, hydrocarbon failures can also be caused by oil burning due to worn valve guides, valve guide seals and/or rings.

P0420- could be the catalytic converter or the oxygen sensor/s switching activity

Bosch Platinums? Worst plug on the market. Put the originals in it